tessb:
You need to be careful claiming for the items worth £75, you're only covered for I think £36 with Recorded Delivery, and I think if you say the item is worth more than that you might not be entitled to anything, it's like under-insuring something - check before you claim, it might be worth just saying it was was £36 otherwise you run the risk of getting nothing back.
Call in to your local post office and have a chat with the counter staff, they're usually really helpful and know how to work the system!!!
Thanks for your help! They didn't know at my local post office I'll be really annoyed if this is the case! I'll only have ebay proof that it cost £75 and a copy of a letter from the seller telling me to return it! I'll keep my fingers crossed that it turns up.