Turns out it wasn't the battery after all, but the earth connection.
I have a quick disconnect thingy on the negative post, unscrew the handle and it disconnects. Anyway it looks like at some point it was unscrewed a little but not quite enough, and there is evidence of sparking and the like, and there was a carbon build up leaving a bad connection.
Cleaned it up, and electrics are back to being good.
Makes me wander if I was too hasty throwing the old battery now.