If that does not produce any result then look at opening an Ebay dispute

Can someone point me in the right direction with this.

I bought some trainers for £24.99 with Free P&P.

The trainers came but were not what I expected, so I returned them to the seller, as agreed through his return policy.

My seller then refunded me £20.99, I asked about the other £4, and he said that's what it cost him to send me the item so he's keeping it.

Non of this was stated in his return policy, and I thought the P&P was free.

*UPDATE*

Opened a dipute via PayPal within an hour the seller had refunded me the retained £4, and stated in his email "It's not worth the headache".