o2 Broadband

I have been with o2 broadband since may 09 and an 18 month contract which the girl in o2 store recommended @ 19.99/month Every thihg was fine till the last bill which was 105 euro, I was told when phoned o2 about this they told me I went over the agreed amount of usage,i cannot understand how the bill can increase by over 500%.I was told iused 15mb instead of 10mb. Is this possible and if so why was I not told this before I cannot afford to pay this amount ,what can I do ?

A very quick search of o2's website showed that the excess charge is 1.6c+vat per MB (€20 a GB),so the charge appears correct .Also,the site contains an explanation of how to check your usage.