The power supply was bought from a reputable astronomical shop (geez, that narrows down the seller doesn't it!
). It is marketed as for the SW mounts, and may even be Synta made (not sure). Cost was $65 I seem to recall.
The culprit was the power lead IN. This is one of the desktop PC type power cables - I have many spares, but not sure I trust the whole thing anymore!
First thing I did was check the mount, and thankfully all was fine.
I'll do photos in a second - been a busy day today.