[Diabolical Artificer]

Thanks for those pointers Frank, my method does seem a bit over the top when you think about it.

I've had part success in finding the fault. I started to take voltage readings of everything and whilst doing so I leaned my arm on the chassis, fault stopped. Aha! So it looks like I have an intermitant joint or connection somewhere. My intuition says it's a valve base or similar.

I'll take a video of the scope trace tomorrow, it's a weird one, one I've not come across before.

Andy.