[Arran]

Very likely that a bad mica is the culprit, either as a coupling cap between the oscillator tank circuit and the tube or a cap in the tank circuit itself. Calling the oscillator tube and oscillator is rather a misnomer, the tube does not oscillate the tank circuit is what oscillates, the tube just provides a kick of voltage at the right time to keep the tank circuit oscillating. If the oscillator coil has continuity then one of the micas has to be the problem, sometimes more then one if they ended up with a bad batch. You can use a silver mica capacitor or a precision ceramic one as a replacement, make sure that you use the correct value and observe where the original leads are routed, not just connected, before you take it out.
Regards
Arran