OK, so this evening I found a few moments to complete the setup of PSU module 5, after its capacitor change. The fault remains as it was prior to the caps being replaced.
Most of the supply rails were pretty much as expected -
3v = 2.9v
6v = 6.0v
12v = 12.7v
24v = 25v (raw battery input)
110v = 108v
121v = 120v
The only thing amiss in terms of level here was the 110v rail, which the replaced capacitors are part of. A careful tweek once the set had settled down put this up to a steady 109.9v and the 121v to 121.1v.
However, there is, I believe, something odd going on with this rail! Monitoring it at switch on, it takes nearly 25 seconds to rise to 110v! During this time, up to about 50v the unready tone sounds, it then sounds momentarily a few times as the level climbs up over 75v, 90v and 100v and up to 110v. Im sure it shouldnt take nearly half a minute to reach 110v!
Something else ive noticed - at present im testing on the bench with no input signal. In this state, once the 110v rail has risen and the unready tone ceased, I get a nice steady low level of noise from the handset, no pops, clicks or static crashes, no sudden noise bursts. Whether adjusting the supply rail has solved that, or whether it is a function of having an input signal present (perhaps an AGC issue?) will have to wait until I can test further with an input signal present.