Armed with the knowledge that the hum could possibly be coming down the signal chain to the grid, I decided to start at the valve and work back. I snipped the grid resistor R27 (Trader 1383) by a tag strip. After switch on, no difference... the snipped lead ends of the resistor had sprung back and remade a contact. I then carefully pushed the gap open with a plastic rod...very loud hum. I opened and closed the gap about 2-3 times with loud popping from the speaker and the hum went...turned the volume up and a lovely clean FM station played hum free!
Could the loud pops caused an internal valve short to be cleared? Could I have 'bopped' the grid in effect?! I will resolder the resistor back or replace it as it has crept up from 4.7K to about 6K when measured. I will report back if the grid voltage creeps up again as before.
I think I will retire the contact cooled selenium rectifier with a silicon diode and resistor on a tag strip and get the HT back up to full spec with some wirewound resistor trial and error.