Hi Damien,
assuming there's been no frauds, you haven't used this data abroad etc, then it would help if you had a detailed bill of the dates and times these charges were applied, and the quantity of data involved in each session so you can try to remember what you were doing on your phone during each one of those dates and times.
If you were on an older O2 tariff that had not changed/upgraded for a long while, you probably were on the older data charge scheme where it was charged per MB, so these charges would not sound outlandish with moderate to heavy use. Newer tariffs (since the beginning of 2008) have a cap of £1 per day (subject to fair use etc).
All in all, unless you were downloading a movie or two on every single lunch break you've had recently
, and of course excluding the possibilities above (data abroad etc), I don't see how these charges could stand.
If taken to court and fully defended by you, they would probably have to show some sort of evidence from the logs of the mast(s) you accessed when you used all this data, which would show where this access took place from and (hopefully) acquit you (esp. in case of fraud/SIM cloning etc).