Thanks so much, Nick and Dave
With regards the multimeter, I have just ordered one so will have to wait until that arrives before I can report back.
I replaced both the line and headset cables as well as the 4T earpiece speaker and the no. 16 mouthpiece transmitter (replaced with a 21A electret transmitter)
I’ve checked through these steps with the following results.
1) After removing the click suppressor, the line level is greatly increased and I definitely hear myself blowing in the earpiece - this stops when I press the black buttons for the handset cradle. After temporarily disconnecting the grey wire from T1 there is no change.
2) The PCB looks good to me!
3) See above re: multimeter.
4) It's really quite hard to see but I think there could well be some broken/missing copper wires. If so, is this something that one might be able to fix themselves?
Thanks again