Are they liable for p&p?

There's not enough information in the question to be able to say for sure.

However, its likely that there were two contracts. One with PooCon for the purchase of the voucher in exchange for cash. And another one with Supplier for the purchase of the goods and p&p in exchange for the voucher and a bit more cash.

When you say the "deal fell through" I assume you mean that Supplier did not fulfill your order? In which case that contract was breached or it contained terms allowing them to cancel it. In either case they owe you back everything you paid them. For the purposes of this contract the fact that part of the payment was a PooCon voucher is irrelevant. Supplier still has their normal contractual obligations and you are still covered by all the normal consumer protection rules. so no specific examples are required.

I assume the PooCon contract offered some kind of guarantee that Supplier would be able to fulfill the order? So as they couldn't you then have a right to a refund for the voucher from PooCon, which they have given you.

Having said all the above, it could be that the precise wording of the agreements actually mean there was only on contract, either with PooCon or Supplier. But either way, one of them is liable to refund you in full.

See http://www.telegraph.co.uk/finance/newsbysector/me...

Forget any small print you are supposed to have signed up to. The site should refund p&p if the deal fell through, since presumably the 'seller' incurred no costs?

yes keep ur postage receipt

Indeed.