[dazzlevision]

Despite lacking any scope probes that are really suitable for SMD work, I did manage to attach it to the two clock pins of the tuning controller IC (snippet of circuit attached - CL1 & CL2). The clock frequency is around 160kHz and given that the whistle disappears when the probe is connected (capacitive loading), I reckon it is actually higher than 160kHz, so perilously close to 198kHz.

I’ve also established that the relative orientation of the unit and the loop antenna, relative to the direction of the Droitwich transmitter, does affect the whistle.

I’m confident all the grounding screws that fix the PCBs to the “chassis” are tight and all lead dressing is as originally found.

So, maybe I’ll try reducing the tuning controller’s clock frequency a little - but hope it doesn't affect its correct operation. On second thoughts, maybe I should first test the Radio 4 LW reception with the metal outer case fitted - but it’s plastic!