Adding a small bypass capacitor across the supply, in this case a 22nF polyester that just happened to be loose on the bench, made no difference to the flickering LED problem (which I discovered also manifest as a low rapid clicking from the loudspeaker!). So, I thought to try it instead as bypass across the G-M tube cathode resistor.
This solved the flickering problem, but created another - no more pulses!
Clearly this was absorbing the pulses without triggering the indicator circuit. So I moved it instead to bypass the base of the 2nd transistor - bingo!
Good, solid flashes and clicks with each pulse, but no more dim flickering!
22nF might be too high a value, so at a later time I will try and find the lowest value that gives good results.
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