The suspects: who is Joanne Davidson related to in Line of Duty?

Line of Duty ended on the mother of all cliffhangers on Sunday. Never mind the thrilling shoot-out scene, viewers were left with one question as Ted uttered his favourite catchphrase, "mother of God", and the credits rolled. Following a thorough search of Farida Jatri's (Anneika Rose) flat, the AC-12 team had found some very telling DNA samples.

When Steve Arnott (Martin Compston) walked purposefully into Ted Hastings's (Adrian Dunbar) office, he laid out the details for us. They had found Joanne Davidson's (Kelly Macdonald) DNA in the flat – and although Hastings looked pleased as punch by this finding, it was hardly a surprise for us. They'd been in a relationship after all. But there was more.

Another partial match had been found from Davidson's DNA. And then Arnott dropped his bombshell: "Now as we know sir, there's only one credible explanation for Jo Davidson's DNA matching this individual. They're blood relatives."

Arnott was explicit about the DNA match belonging to a male. So who could this person be? Let's examine the suspects.

Tommy Hunter

The prime suspect, by dint of his Scottish accent, is this organised crime gang member who was killed back in season two. It was Tommy Hunter (Brian McCardie) who recruited a teenage Dot Cottan (Craig Parkinson), who had been his caddy when Hunter played golf, to the OCG. We first saw him blackmailing detective Tony Gates (Lennie James) in season one and he was a paedophile who ran a crime ring abusing young boys in Sands View children's home. And it was Cottan who eventually arranged to have Hunter killed after Hunter became a police informant to keep himself out of prison.

He might have recruited Davidson too, like he did Cottan. But it's clear that Davidson is not at all happy about being a bent copper. She's told the sinister chatroom that she's done – and looked like she'd been stabbed in the gut when Ryan Pilkington (Gregory Piper) told her that Jimmy Lakewell (Patrick Baladi) had been murdered.

Presumably there's a process that police officers have to go through if one of their family members happens to be one of baddest men in town, if they're even allowed to join the force at all. And Davidson has insisted she doesn't have a family while also angrily throwing a glass at a picture of her mother – so we know that's a porky pie. Could her brother, cousin or uncle (he was only 14 years older than Davidson, so too young to be her father) also be the sort of person that makes her rage-throw kitchen items? And could someone be blackmailing Davidson over her blood ties to a crime gang that she wants to keep hidden?

Lee Banks

A hardened member of the OCG alongside John Corbett (Stephen Graham), the man who killed Maneet Bindra (Maya Sondhi) in season five and who, minutes before the end of last night's episode, strangled Jimmy Lakewell to death in front of a terrified DCI Buckells. Sent to prison in season five, the last we had previously seen of Lee Banks (Alastair Natkiel) was during an enigmatic visit from Ted Hastings, who told him firmly that he had something to say to Banks that he would want to hear. It's never been stated explicitly, but surely it's not a coincidence that he shares a surname with Carl Banks, the murdered criminal who was swapped out for Terry Boyle by the OCG.

"You watch what happens to a rat," Lee said menacingly to Buckells as he yanked the wire around Lakewell's throat. It neatly mimics the words that Davidson herself once used to describe Jatri, and which, moments later, was repeated by Ryan Pilkington to Davidson. It's hardly an uncommon phrase, and Banks does not have a Scottish accent, but was its juxtaposition shortly before the final scene meant to be meaningful all the same?

Jimmy Lakewell

Season four's legal smoothie returned as the mysterious voice on Gail Vella's podcast, and is now stone cold on a prison floor. We first saw him as the lawyer friend of Roz Huntley's (Thandiwe Newton) husband and working with the corrupt copper Derek Hilton (Paul Higgins). At the end of the season, Lakewell was sent to prison for perverting the course of justice, which is where Arnott found him.

Though it sounds like Lakewell never passed any information to AC-12 on the record, he must have whispered at least one key fact into Arnott's ear based on his not-remotely-subtle post-interview comment about not telling them anything. Whether the OCG just thought he was a rat or knew for sure that he was one, we can't say. But could he be related to Davidson? Her distress at the news of his death could have just been her fearing that the same would happen to her. But it could have been the reaction of someone who's lost a family member.

John Corbett

Is it possible that the man that Davidson is related to is a fellow copper? As Arnott explained, police DNA is kept on the system so that it can excluded from the results of a crime scene. But this DNA was found by searching the whole system. "The analysis detected partial matches to a nominal whose DNA is stored on other police databases," said Arnott. Which, we assume, means the relative is not a copper. A "nominal record" is for people who've had convictions and cautions. But could the DNA of coppers that were arrested themslves be included in this other database?

Poor John Corbett (Stephen Graham), the undercover detective who got in too deep and was committing crimes alongside Lee Banks and Ryan Pilkington (Gregory Piper), was killed in season five. If he and Davidson were related, it could mean that the mother in the photograph is Corbett's mother too – and at a glance they do look similar.

But, as we know, Corbett's mother was killed by the IRA in 1989 and he was adopted by his aunt and moved to Liverpool when he was a child. If they had gone their separate ways as children, then it would explain their very different accents. But why would Corbett's aunt not have adopted her too? Nor Hastings have any knowledge of her, when he knew Corbett's mother well? Even their birthdays are only a few months apart, making the sibling link impossible. Corbett may have been a bent copper, but he's unlikely to be the DNA match in this case.

Danny Waldron

Danny Waldron was the shock death at the beginning of season three, shot by fellow policeman Hari Bains on the order of Dot Cottan. Like Corbett, Waldron's mother died when he was a child and he moved to the north of England to live with his father and stepmother, before being placed in care at Sands View Boys Home. There he was sexually abused by Tommy Hunter and his gang. In Waldron's police interview, Arnott stated that Waldron had no registered next of kin. So it's unlikely that he had another sibling – and if his father had had a daughter, her accent wouldn't be Scottish.

There is one small detail, however, that could link Davidson and Waldron. In season three, his address was 5 Croxford Street. Davidson also lives on Croxford Street. Could this be just a coincidence?

Dot Cottan

Another broad northerner, little is known about Matthew Cottan's upbringing, aside from his being groomed for organised crime by Tommy Hunter when he was a teenager. Edge Park golf club, where the two met, is located in the unnamed city in which Line of Duty is set – so he didn't spend his teenage years, at least, in Scotland. He rose through the ranks of the city's police force and ended up in AC-12 as a detective inspector. But his links to organised crime are now well known – and it is on his information that AC-12 believe that there are multiple senior corrupt policemen and that H is not just one person.

If he were, somehow, related to Davidson it would certainly warrant the "mother of god" exclamation from Hastings. Being linked to him would certainly be embarrassing for Davidson nowadays, but he wasn't exposed until season three and Davidson has been in the force for years. Her claims to not have any family would be strange if her own brother was also working in the police force, in the same city.

Mike Dryden

If we continue to follow the clue of the Scottish accent, that leads to two further places. One is Mike Dryden (Mark Bonnar), the former deputy chief constable of Central Police. He may be serving a prison sentence for perverting the course of justice (for lying about getting a speeding ticket in order to have an alibi for engaging in a sex act with teenager Carly Kirk), but he's hardly got the same links to organised crime as others on this list. And, similarly, it's unlikely their connection would be a secret given Dryden used to hold such a high rank. Like Hunter, he's also 14 years older than Davidson, meaning they wouldn't necessarily be personally close if they were siblings. But it's the secrecy that makes Dryden an unlikely suspect.

Derek Hilton

The former assistant chief constable of Central Police is the second bent copper to have a Scottish accent. Another senior figure to have links to organised crime, he ended up shot on the end of a jetty after he was named by Roz Huntley as the person who ordered Robert Denmoor, one of the balaclava men, to throw Arnott down the stairs. His death was ruled as suicide, but, as Kate Fleming (Vicky McClure) pointed out, that jetty had been used before as the scene of a murder that was made to look like a suicide.

He, too, was 14 years older than Davidson (1965 was apparently a big year for the birth of future high profile police and criminals). But little else is known about his background, so there's not much to link them. And, like with Dryden, there's no reason that she would keep her link to such a high-profile member of the force a secret.

As the most H-like of possible Hs, Hilton being her relative certainly would be a big reveal. But given he died before he could be charged with a crime, it seems unlikely that his DNA would be in this database for criminals.

Conclusion

Based on the evidence, the relative is not a cop and not a woman. With a Scottish accent and a habit of drafting in coppers to the OCG, the smart money is on Tommy Hunter. But Jed Mercurio always likes to keep his viewers guessing, so don't rush to the bookies just yet.

Have you spotted any other clues? Or could it be Nigel Morton, Lester Hargreaves or Patrick Fairbank? Or someone else entirely? Leave your thoughts in the comments.