I've had letters returned to sender, emailed the buyers and got no response.

If they open an INR case you will have Paypal protection as you can provide proof of postage to the buyer (presumably). And could possibly get a neg overturned.

I would message the buyer through eBay asking them what they want to do. If they tell you to resell it, refund them minus ALL associated costs - postage, FVF, listing fees. If they want you to resend, charge them for another lot of postage, as you will have to pay that to resend.

Block them.