It got stranger....but in a good way!!
I didn't get a chance to check the USB port but will consider to swap it out with a good unit cannibalised from another identical laptop which has a bad screen.
Anyway, I removed the battery & ran the bad unit successfully with power connected.
I then put a good battery in to the bad unit & it ran OK. I also ran that unit with the good battery & no power connected.
I then put the original battery from the bad unit in to another laptop & it ran OK.
I put that original 'bad' battery back in to the original 'bad' laptop & everything is now charged up & running OK!........
Maybe there was a safety cut off switch in the battery to prevent damage, & just needed to be 'reset' somehow.......?
BUT it was your suggestion to check the battery that gave me the right path to check & resolve the situation........MANY THANKS!!