I stupidly took my rotor arm off the other day and forgot about the whole thing lifting up and dislodging the bob weight springs. I put them back on but strange things started to happen:- cronic misfire between 1200 -2500rpm, no engine braking. hard starting. It nearly messed up a good days trialling....
Anyway I ordered some new springs for the good guys at SimonBBC and then found the reason for the strange effects.....one of the springs had got bent and was catching on the plate meaning it either stayed on full retard or got uncaught and went to advance as the revs went up. Learnt my lesson

Yep, been there!

Well - spoke too soon. It still misfires when it is hot but only under 2500-3000 rpm. Could this be the amplifier playing up?

COULD be ####innnnnnnnnnn anything, including lean when warm.

Amps can be heat related yes.

Hotwire system working good?

I've just done the voltage tests for the coil etc and they work out ok so maybe it is fuel. Running SUs so not EFi but carbs were rebuilt a few months ago so I don't think it would be that.