I think I am going to conclude my sidetone is within normal limits. The main components affecting it seem to be R1, R2, C1B, the regulator and the transformer. Main suspects would be the resistors and capacitors, but the Rs are close to value, as the is cap, and the cap' has no obvious leakage path.
I have however fixed a minor defect... In reply 36 to this thread, I mentioned the nostalgia of
'hearing the dial pulses'. Looking at the circuit diagram, they're not meant to audible - the receiver being shorted out by a pair of dial contacts when it's off rest.
A brief and gentle wipe at the affected dial contacts with abrasive paper and a coating of switch cleaner has rendered the pulses near-silent, as they should be. Don't ask me why they are not
totally silent, I guess the dial contacts are not quite achieving 0 ohms. But I do seem to remember it was not unusual to hear very faint pulsing in the receiver when dialling, so I think I'm going to leave well alone.