I completed one of these deals last year without any problems with a Virgin contract. The pages of the Virgin contract that I would receive clearly showed that I had paid the bill.
With my O2 contract, all I get is a single page titled Your Bill which shows the mins I used last month and amount due this month including the date that they will debit my account.
Is that all that I need to send to claim my cashback? It doesn't seem like it has all the info they asked for when I was on my Virgin contract... but it's all I ever get from O2?