i got one on the same date as you 15th august, im not happy and i defo 100% never got a txt off anyone, iv not spoken to o2 yet but has anyone spoken to them to get this credit back on there o2 bill ?
right according to the person i spoke to with o2, the charge comes from a company called Bango who seem to deal with mobile internet payments. http://bango.com/mobilebilling/
o2 credited my account with £3 so it's worth getting in touch, but they couldn't tell me what i had purchased for the £2.55
Bango say they deal with charges for downloads and apps, and seem very aggressive in the way they take money so just be careful when using mobile internet.
i hope this will help some people who come across the problem in the future.