OK.
Thanks to all for their sage advice,
That set up that joeqsmith has is seriously impressive
The discussion gave me the incentive to get on and do this thing and investigate rather than put up with it.
I was considering the probe route to have a look at the waveform on the HT lead, and was thinking how to liberate the relevant equipment from work, but then considered what I wanted to achieve if the investigation was successful.
I don't think my popularity was going to improve if I wrecked any of the work kit, and the CRO I have at home is an old friend, and I'd be pissed at myself if I managed to zap it.
My thoughts about this being a job for some parallel capacitance as suggested, and relatively easy to implement for a first (cut and) try.
I'd found some close weave braiding in the waste that was stripped from some coax and used to sleeve the bikes previously removed HT lead to the plug with. I didn't so far measure the core to braid capacitance which I should as it may give clues to what's happening.
I switched on my air band radio, (can't find the PMR one at the moment!), and started the motor. The unscreened cable made the radio swamp with ignition noise.
With the "screened" cable however and with the braid clipped to the head, the level of noise was reduced to the point that it was possible to hear the alternator whirr on the radio as well: the difference was not markedly great in volume, perhaps 6db less, (a quarter), but that didn't take into account the effect the AGC would be having in the set as well.
@ Circlotron...
Both times I tried with, then without a randomly chosen (that fitted) clip on core from my junk box. Really I have to say no difference.
Nowhere near perfect, but a Crusty the Clown approved good enough for me.
Thing is, this was just purely empirical research... I now
want to see those HT waveforms more than ever...