Well ive finally tracked down the odd 'silent' interference affecting 2m when im mobile -
Its the supply to the front dash camera!
I suppose I should have realised, it is after all only a cheap Chinese USB switching regulator. But, the way the level and frequency of the interference changed during a journey had me thinking it was something else.
It came to light that this was the cause after some experimentation over lunch. No interference with the camera off but ignition on (regulator powered but off load); no interference with the camera on but on its own battery (regulator not powered); but ignition on and camera on (regulator on load) = QRM.
Ive just dug out a bag of snap-on ferrite beads, TDK type ZCAT1325-0530. From what I can interpret of the datasheets, these are intended for EMC filtering on USB cables. Ive no idea if these will provide enough impedance, but they are all I have so i'll have to try them.