I spent a couple of hours looking at the audio section of the MC receiver and discovered that AVC really meant that and not AGC in respect of RF.
Whatever input, both RF, and audio via the gram input, resulted in a constant audio output. When I'd tried peaking various trimmers, RF and IF, nothing seemed to happen at the output other than the odd transient change before settling back to constant output.
The very strange circuitry around a couple of the audio amplifier valves, including the use of a 6B8 is to arrange a steady output amplitude. It allows intantaneous changes to deal with the audio waveform, but over a second or so the output reverts to something like 2.5 volts RMS (for a constant tone input at almost any amplitude).
The output waveform is very distorted, having a second harmonic distortion shape. If the input level is reduced there comes a point when the output is distortion free then it drops following the input.
Whether this distortion is a side effect of the constant level circuit or one of the components not up to scratch or a missing wire I haven't yet discovered.
Maybe the circuit needs looking at and comparing with similar circuits of the same vintage?
http://www.radiomuseum.co.uk/moreton2.html
The volume control appears to work normally.
Allan