If you have no tracking number, there's not much you can do. You'll have to refund the cunt buyer and claim the 'loss' (up to £20 I believe) from Royal Mail if not sent tracked/recorded.
Royal mail have a minimum amount of time before you can request a claim. 15 days or something like that. I'd wait until that has passed before refunding him. Its only been 5 days so a bit hasty for him to open a claim.
If you have no tracking number, there's not much you can do. You'll have to refund the cunt buyer and claim the 'loss' (up to £20 I believe) from Royal Mail if not sent tracked/recorded.