Check the email they send you with your bill amount on, it also explains on the email that the first bill you get will be more than normal.

Ive had my online contract for over 7 years now, haven't actually logged on to view my bill in detail for about 5 years as I cant remember the log in details, but my bill has always been spot on to the amount i've used it.

Cant fault them to be truthful, but then i've not had to speak to them for years so dunno if thats good or bad lol