Well, seems my next step is to STFU and take the hit.
You apparently only have 21 days from the moment of returning the item to advise eBay that you've not had a refund. After that, it's a case of 'go fuck yourself'
It's only £20 so not the end of the world, but it's the principle. It's a business seller, too (495k eBay sales). They've ripped me off and got away with it.
I'm fucking livid.