UPDATE: My batteries arrived today. I replaced the battery in the one Minimote that has never held a charge and so far all seems good.
I left the exiiting wires soldered to the board, clipped them then soldered the new battery to those. the wire on the replacement battery is not long enough to reach the solder points on the PCB.
When I charged the new battery up it never got warm (like my others do very slightly) so for a while I was concerned that perhaps it wasn’t charging. The again, I did have the cover off still. But after an hour or so the light turned blue.
I’ll report back after a while.