Selfsufficientish.com

What can I do with two used up laptop batteries other than take them to the municipal tip?

Nothing, as far as I know. Just take them to be be disposed of properly.

What about a nice pair or earrings

I wondered if there was an organisation that dealt with them, or special bins somewhere. I'll check with the Council. As for making earrings, they are two different brands and a wee big heavy for my delicate lobes.

Apparently PC world take them free of charge as well as quite a few big stores,

Not helpful for you but most computer shops here will take the old battery when you purcahse a new one - as well as the old computer, printer, printer cartridges etc. There is also a National Computer and TV recycling scheme operating through charity organisations (usually providing employment for disabled) whereby these can be dropped off at specific sites for recycling.

Here's a few ideas though in all honesty there doesn't seem to be a great deal that can be done with them ,
https://www.wired.com/2011/09/18650-thi ... p-battery/
and similar ideas here,
http://www.instructables.com/id/7-Ways- ... p-Battery/
Seems to be the only real option is to take them apart and reuse the cells that are still ok and judging by the comments about their chance of catching fire maybe the ear rings might not be the best idea.

I've been collecting ordinary used batteries for a couple of years in a specially-provide bag. When I had a look to see how it was filling up I could see that some of them had become coated in something, will they still be useful for recycling, I don't use batteries a lot so I wasn't keeping a frequent eye on them.

The "something" is perfectly natural or rather usual would be a better word . Here's a bit about it ,
https://modernsurvivalblog.com/preps/ba ... revent-it/
It doesn't say if it affects their recyclability but I've seen some very rough looking car batteries at my local scrapyard that are still worthwhile to process so I would suspect your batteries can still ecycled.