OK, what you do is go to the Post Office and get a claim form, fill it in, attach the proof of posting - but the Royal Mail won't investigate until the delivery is 28 days from the date of posting.
At the same time, write to the buyer telling him what you've done, saying you are following standard eBay procedures for un-delivered items, and don't let him pressure you.
If he opens up a dispute on eBay, you have the proof that you are looking into it with Royal Mail, so make sure you keep those copies.
You certainly don't refund until you have been down every channel for investigation - and you don't leave feedback until the whole thing's finished. If he leaves you bad feedback in the meantime, you can complain to eBay and ask them to remove it, as you are doing all you can to rectify matters.
cath, you are wrong that once you have PoP, it's no longer your responsibility - the seller has to instigate any claim for non-delivery with Royal Mail.