[Chas]

Is it just residual DeOxit that could be affecting the pot and if so can it be flushed out with a more standard contact cleaner that is fast drying/evaporating or is it just plain shot?

You could give it shot  

If it works then save all that (cumbersome) troubleshooting...

I am guessing the speaker frame has the output transformer... If the set was in a damp environment, transformer bobbin covered with fuzzy from mold there is a good chance it may be leaking from primary to the core.

If the speaker can be electrically/mechanically lifted from the chassis try that and see if there is B+ present. DO suppose there is a leak, as miss-handling the speaker with a leaking transformer can give a very unpleasant shock and possible muscle reflex damage to the cone.

GL

Chas