OK, I give up. New battery pack I think. I got inside the pack and all there was a 5A 75deg C over temperature bi-metal strip. It showed closed, so that wasn't the issue.
I've given it another "external" charge and put it on the charger, all it sees is a battery and doesn't want to charge!
My "posh" rapid charger has also died :-( Awaiting a replacement! When it comes I'll let you know. This all started when I flattened it in the disc cutter!
The tools are all Ryobi. The battery chargers are the bottom two in the message #31. The middle one has recently stopped working :-( The Battery is made by Dero7eno.