go to the post office and ask for a claim form. If you did not get proof of postage when you posted it then you can’t claim for a thing.
If you have not taken any extra insurance then you will get no more than 32 quid, unless it was special delivery which IIRC has a £500 insurance ceiling.
fill in claim form and attach receipts etc as required. Send off to RM and hope for the best.
I had to do this a while ago and got the money back after about 3 weeks. Print off copies of everything, i.e.the page on the classifieds that detail the sale, print off any email correspondance regarding the sale, include the original receipts for the brakes. Drown the buggers in proof/reciepts/correspondance so they cannot argue or claim info is missing.
Worth noting they put a time limit on replying to correspondance (10 days I think) so you need to reply to them asap once they have contacted you regarding the claim.