Following my discovery of the actual source of the interference on 2m mobile, I found a few moments yesterday to install the ferrites.
A pair of snap-on TDK beads at the camera end of the cable made no difference. So definately an issue with the switching regulator. So it was off with the glove box, on a cold and rather wet afternoon. A couple of beads installed on both the output and 12v feed cable of the regulator module, and the QRM has gone.
Its quite surprising how many signals this was masking! A few quick air tests with M1BBV proved everything to be working well again.
The development of the hardware for the Clansman Remote project is coming, hopefully, to a conclusion. I have Arduino Pro Mini's which will physically fit the space and will operate at 3v. I have 3v I/O expanders, and I have an advanced RS232 line driver coming. However, the I/O expanders and the Line Drivers are SMD, SOIC28 and SSOP28 respectively, and so im now waiting on delivery of 'break-out' PCBs on which to mount these!
In the meantime, the box for the Theremin has finally been acquired! So I hope to get that finished once and for all very soon!