I've gonoe back and checked all my bills since I got the phone in December
The usage starts on the 12th or 13th of each month.

I'd rather not have to call them at all, I'd like to work out exactly how much I'd have to send them as the final cheque beforehand, once I cancel.

Do I have to give notice, or does the cancellation start on the day the letter gets to their office?

if so all I have to do is make sure to check my unbilled balance on the day I send the letter and just send the cheque for any data use (the £5 I will get charged) and any texts or calls not covered by my free allowance?

I presume I dont need to send the next months line rental as its paid in advance?