Hi mate thanks for the reply. Just got back in from spending most of the day on it (and trying to sort megasquirt, bloody idle settings...).

Turns out I was being a right Wally. Because I'm using powder coated brackets the thing wasn't grounded. So I made a cable up to connect it to the block. All working now! I think I need one of those surpressors on it but don't know how critical that is.