Hi Mark I came across the following in the manual and had the 1st AC try this setting but it didn't seem to make a difference to the interference.
Worth noting though !
The CINE RT system incorporates advanced algorithms to ensure an absolute minimum of audio interference. Engaging Sonic Saver reduces the volume level of the BASE ultrasonicsignal. For use only in the extremely rare instance that a microphone picks up audio interfer- ence from the BASE or BUG(S).
Press the SONIC SAVER TOUCH KEY to engage SONIC SAVER (and see the sound recordist smile for once).
Important: Reduced ping volume will result in reduced effective range of the BASE sensor.
Note: This button will display a green border (and a smiling Sound Person) when Sonic Saver mode is engaged.
He also tried different RF channels but no difference. The strangest thing to me is that it only affects the HM (s) not the UC400A all other things being equal. I guess the UC400A case may be a slightly better shield.