This has been asked before..

[#1]

..but can not find the answer:).......It looks likely I will be going over my 10GB limit as i am now over 4GB after only 10 days which i must admit has surprised me compared to when I was with BT. what happens when I go over it and how will I be charged, will I be warned that I am approaching my limit.
Thanks in advance for the answer

[#2]

I think the product FAQs here answer most of your questions and explain the options available. If you choose to pay for extra usage, which is the default, the charge will be added to your next bill.
An email should be sent to the address on your account when usage reaches 80% of your allowance.

[#3]

Thanks for not saying RTFM
dick:quote