[EdHolland]

Some possible insight today. I didn't get long with the radio, but had a moment to switch on and poke about, literally, touching low voltage signal points in the front end and IF chain. Twice, I was able to surprise the set into a state of improved sensitivity, suggesting that something is either unstable, or there is an intermittent contact.

First thing will be cleaning the switches and valve sockets again, rotor contacts on the tuning cap etc. then another look at some voltages. Helpfully, even though I am schematic-less, valve data sheets give a good idea what to expect, plus example circuits and component values.

Something is slightly amiss, but it is playing hard-to-get. It is times like this where it is good to be doing this as a hobby, rather than for a customer