Final (hopefully!) addendum to this thread. This radio has always had a rustling noise in the background on MW/LW, sometimes there would be an odd phasing then back to normal or random pops/clicks. I, in my ignorance, thought initially it was simply down to the age of the valves and/or coils, after all it is nearly 90 years old having been manufactured in around 1933-34. To have some noise would be forgiveable BUT it was irritating! Then I started trawling through this forum & others for the possible causes of it. It took many hours of reading but finally a trend began appearing especially in America of all places. It would seem the mica capacitor is suffering the effects of time. I then found a paper published in 1960(!) giving a LOT of detail as to their failure modes, one of which is silver migration & inclusions in the mica itself. I'm not at all sure if the document is still within copyright so I have not attached it here. The link is:
https://www.google.com/url?sa=t&rct=...10e6UqL2TWJLvf
Anyway, I was becoming more convinced this was perhaps what was still wrong with the Defiant. I remembered I had left one mica capacitor in the aerial side of things. Out came the chassis, it looked good, tested ok as well, bang on value. Grab a replacement from the stock of caps, out with the old & in with the new! Switched on, the rustling/popping/f*rting had gone! It sounds so much better.