Thanks for the replies, as ever. The car has Pertronix fitted, and the timing is bang on, so I don't think either of those are affecting the lumpy idle. I think it's vacuum leaks from the inlet seal things, which I am trying to sort.
Good call on the engine and 'box mounts though..they are next on my list. I am sure they will help reduce the feelings induced by the lumpy idle, but it is definitely not quite right. I like the idea of disguising the idle by smoothing out the mounts and engine shocks, but I like the idea of a smooth idle even more...I need to hunt down those 8 inlet manifold/plenum(?) seal things.

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1971 W111 280se rhd coupe (The Duchess)
1959 Bristol 406 (Monty)
1970 Volvo Amazon 131 (Lola)
Lives in Cornwall, UK.