Item received that I sent back but no receipt from seller claimed

[nothin_2_lose]

I purchased a set of spark plugs off ebay ( or at least I thought I did ) when it arrived it appeared it was a single plug not FOUR. 

Anyway he tried to sell me the other three while I opted for the refund. ( one plug was 10 pounds so four plugs obviously would have been 40 which is excessive ) 

My impression is he shouldn't have put a picture of four plugs & listed only spark plug for sale ( no where in the description did it list it was one it just didn't have the 's' on the end of the plug. ( title ) 

Anyway I digress , I sent the item back to him & a asked for recorded delivery but on checking the receipt it appears I only have .... 'certificate of posting'. He claims he hasn't received it too... I sent the item on the 28th first class so it's obviously not in transit anymore. 

What are my options... I'm afraid I've lost both the item & the 10£ ......