How do I claim back my £10 which he has withheld dishonestly?
Sellers can't "withold" money from returns. Only eBay can authorise this, if the seller is a registered business seller and has satisfied eBay that the itme was not returned in the condition supplied. This corresponds with business sellers' legal right to reduce the sum refiunded under these circumstances.
If you returned this item correctly, by using eBay's money back guarantee, eBay will have issued the refund and if they have reduced it by £10 you may be able to appeal their decision. If any queries you will need to contact customer support.
You don't even mention using eBay's money back guarantee? If instead you just you just tried to negotiate a refund direct with the seller it will be between you and the seller. If you're within the 30 day time limit to use the MBG and you have tracking proof of returning the jacket you could possibly still return any refund made privately by the seller and open a case for a refund through eBay.