Two thoughts:
Firstly, if the credit was part of the phone package you might only get it when you go through the registration process for the phone.
If not, you need to ask Carphone Warehouse how they processed the credit. If, as is likely, they used a 'pay point' terminal (of the type that newsagents, supermarkets, etc use when you top up a phone) they should have given you a receipt with the top-up registration number on it. If they've actually got a direct top-up facility (similar to that when you top up your phone at an ATM) the machine should still have produced a receipt for the top-up (separate to the combined receipt for your phone & top-up).
Ask them what evidence of top-ups they give to customers, and why you didn't get any.