Well, that was weird. I reverted the wiring, then started to desolder the cover, got bored, as i couldn't get enough heat into it, just hooked it all up again and, bingo! The output is up to what it should be. A small amount of wax had dripped out, maybe the magnetic circuit had been interrupted, who knows?
I'm one happy bunny, as this simple project has been plagued by niggling issues at every turn!