@V
thanks for thinking on this again, the state of play ATM is that the Dakiom made things worse and was quickly removed.
I have contacted the designer, who, since this was a prototype he built some years ago (the supplied hand drawn circuit diagram has a number of differences to the actual circuit as built) naturally has little recall of the actual circuit and no recommendations to make and without 'scope traces could offer no ideas. I suspect he is, quite reasonably, dubious of my diagnosis.
I have changed a number of other components in the front end for better quality, all to good effect.
The issue is much reduced and the overall character much improved, changing all the cathode bypass caps made quite a change for the better.
I have yet to change the power supply 'lytics (finances rather stretched ATM and Jensens are not cheap).
The overall design is cascode input stage with cathode follower driver stage driving the screen grid of a pentode.
A couple of aspects of this arrangement have me puzzled, the cascode uses a resistor connecting the plate of T1 to the cathode of T2 and the grid of T2 appears to take its reference from the cathode of the output tube (through resistors - I also replaced these).
The main feedback circuit to the cathode of T1 has silver micas and I replaced the generic carbon films with Takmans - at least for the time being.
Not being able, ATM, to replace the plate supply 'lytic on the cascode stage, I bypassed them with small polystyrenes, not sure this was a good move, thinking of replacing them with oil fills, though with 10uf those are rather hefty. Still, there is still plenty of room, just have to fit some hangers.
It may be that what I am now disliking is cascode phase distortion or even the pentode output signature.
It may be possible, with more knowledge, to detune/lower the output (it is a deal more than I need) to reduce this, or to vary the load in parallel to the O/P transformer on the plate of the output tube.
More research needed
Will change the remaining caps and replace the input wiring when funds allow and then take it from there.