Ok tracked it a bit further.
Looks like the feed from the transformer also goes to a bridge rectifier mounted on the backbox.
Appears to be an 18 volt DC feed for lamps, feeds the MPU board.
Haven't disconnected the board yet, but if I disconnect the plug to the bridge rectifier it no longer pops a main fuse.
Not sure why it doesn't blow the fuse before the BR   
(yep its the correct size as per the manual)
What does everyone think? Bridge rectifier a dud or might it be the monster cap adjacent to it?
Cap looks OK but haven't tested it.