Ok, it seems that I was a little premature in thinking the problem was beaten.
The ADF sense antenna was replaced with copper cable and within a couple of flights the issue re-appeared.
Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue. Given that the existing ADF sense antenna has been there in one form or another for the best part of 40 years without issue, it simply has to be some failing component that is causing the problem.
My LAME wants to re-locate the ADF antenna to the belly of the aircraft, but I think it is worth replacing the post on top first.
I'll let you know how it goes!