I agree with Murrymints, make sure you go through the itemised bill when you get it!
I've just had the same problem with vodaphone and my angelic 13 year old....Although she did no where near £900....she would not be alive if she had.
I am tied into a contract with her phone until October and when I asked to have her phone 'capped' they couldn't/wouldn't do it, with a little messing about though I got her 300 extra minutes [I know it's kind of rewarding bad behaviour] for 70p. I now pay £17.90 for unlimited internet, Blackberry messaging, unlimited texts and 900 minutes. It's worth seeing if they will change the price plan if you have time left to run on the contracts.
Lisa x