A Key cap has fallen off. How do I refit it?

[Bicko]

@Bicko 
Based on your message and the picture you attached, the plastic hinge is still attached to the key cap. 
Using this reference: 
https://www.computerhope.com/issues/ch001244.htm ...

Thanks hmuller, that's exactly my situation.  I'm reticent to dismantle the key further; I'm not saying I won't do it if I have to, but I seem to remember Lukasz saying not to remove the mechanism from the key - this was way back in August on the Telegram/Matrix thingy. Back then it was a discussion about swapping/moving the keycaps.  I'd love to hear from anyone who has done that before I try to go any further.

I have the same problem with key "A" I can out bck in but it it a bit loose and pops out after a couple of presses.

Not an ideal user experience!
I notice my 'tab' key is sometimes sticking down, on its top right corner, if I don't press it centrally. I really hope this isn't a warning sign.
I love this device, aside from the keycaps issue.