Thank you all for your help. Daisy, that's brilliant, exactly what I was looking for.
They seem to go out of their way to make things difficult on the website, probably because it results in people spending money they didn't intend to.
Trystan, the one I was considering is £12.99, not a king's ransom and as you say, not worth any risks. Thank you.
I'd decided to replace my tatty old one at this time partly because it's tatty but also because I've hardly spent anything for weeks and decided to treat myself. I've actually had the Kindle for four years which is one of the reasons it's proved so difficult to narrow down the choice. There are ones that come up in the filtered list as fitting my device but when I go into the specifications I find that the size of the case is slightly smaller than my Kindle so wouldn't fit.
Ruskie, I read your reply, strangely enough the one I put in my Save and Compare list is a Fintie one. I have just now clicked on the link in your post and couldn't believe it when I saw that it's the very same item! What are the chances of that?!! I had decided to be a cheapskate and go for the black one but I'm dithering now, I might push the boat out and go for a colour! My existing cover was really cheap, it's been to many places around the world and stood up to the rigours of various beaches, trains etc but it's time to say goodbye.
I always check where Amazon items are dispatched from before buying, I would only consider ones from the UK at this time but thanks for the warning. Who knows what will happen to all the cheap Chinese imports in future, many I can happily live without but things like folding plastic hangers for travel come in handy.
I have managed to find a serial number for my Kindle, it was as you say, in Settings. I think I'll just go for the Fintie one, Ill risk it for a biscuit!
Many thanks folks