o2 didnt charge for gprs until around two months ago, they sent all customers an sms telling them charges would start to be incurred in accordance with their tariffs.
2.35 per mb for older tariffs not changed since since b4 sept 2005.
£3 per mb for newer tariffs changed after sept 05
its all in the terms and conditions that gprs would be charged at that rate but since this 'promotion' has ended theres alot of customers who have used the gprs before and not got charged are finding themselvevs with hefty bills like yourself.
unfortunately as you have used the service you have to pay for it.
call CS i'm pretty sure they can knock some money off it for you but it wont be alot.
i feel a payment plan is in order for you but if you arrange one you'll have an outgoing call bar placed until the debt is cleared.
apologies this is not what you wanted to hear but this is the procedure.