Ah ha, I think I may have fixed the idle surge problem (or at least covered it up!)
There's a hidden adjustment screw on the air flow meter (you have to drill out a metal plug to get to it).
Wound it all the way in (about a turn and a half), then out 4 turns and the surging has gone
The air flow meter is a crappy ancient vane flow type and I think the screw adjusts the amount of air that's allowed to bypass it when the vane/flap is closed.
My suspicion is that is wasn't allowing quite enough air through at idle and the surging was something to do with the vane opening and closing. I guess it could be a bit sticky at it's closed point, or possibly upset by me not putting it in the correct orientation.
Whatever the case my bodge will do for now
