Yes it has only been reported in two units so far. So it should not be a show stopper for anyone.
The radio is usable and I will simply add an audio low pass filter if need be.
For a radio aimed at experimenting it is as expected. I am certainly not complaining.
One point of clarification though. The disappearance of the tones within the passband is due to changing the Arduino, not mounting it on extension leads.
So I believe there were two (probably unrelated) issues, one with the Arduino which is resolved and the second one still present.
Thank you everyone for your feedback. I will digest all this and do more experimenting.
73, John (VK2ETA)