[Arran]

My first suspect would be an open cap on the center lug (wiper arm) of the volume control, or at least something realted to that control, I see three capacitors and two resistors connected to it. I guess the first question that I should ask, does the radio still have the molded bakelite condenser blocks in it? What often happened with these sets is that the bakelite blocks would go bad and someone would connect a replacement cap to the terminals of the old block. If someone has messed with it the first thing you need to do is take an inventory of the parts that are supposed to be there according to the factory service info, it looks like N.A has it all there. It would be best to tear all the mods out and go back to the factory diagram if possible, it is never worth trying to trace out what someone else did.
Regards
Arran