Bad news, the local fix-it guy couldn't fix-it. He simply said it was ok, the voltage coming out
of the charger was good and the voltage coming out of the battery was 36v and "pretty good too".
I gave up trying to explain the battery should be 42 v after a successful charge and the charger usually spins its fan up
to dissipate heat during the 3-4 amp charge. Which it doesn't now. So I rode it back to work watching the 3 bars on the display dwindle to 2.
Plugged it in at work and it still doesn't charge. Rode gingerly home on level 1.
I still reckon it's the bms motherboard and I'll have to find a bms from somewhere but in the meantime I took a leap of faith (since I've never bought from them before) and
ordered a 17Ahr Panasonic 18650 3400mAh bottle battery from Aliexpress:
https://www.aliexpress.com/item/Free-sh ... 82185.html