Sky Mobile's plans are relatively easy to understand. You can either pay £10 per month for unlimited calls and texts (free for Sky TV customers), or 10 pence per minute or message on a pay-as-you-use basis. Rollover data and other perks are available to everyone, meaning your main concern is choosing how much data you might need for that particular month. As it stands, 500MB is £5, 1GB is £10, 3GB is £15 and 5GB is £20. From this Friday (September 15th), however, Sky is increasing the data allowances on the top two tiers: £15 will buy you 5GB, and £20 will get you 10GB to play with.
As Sky Mobile lets you jump between different plans each month, these higher data caps are available to both old and new customers. It's pretty common for mobile providers to fiddle with their tariffs several times each year. No points for guessing why Sky Mobile might want its plans to look particularly attractive on new iPhone day, though.