Short version: Replacing the HT rectifier brought the HT up but made no difference to what was on screen. So by doing that I'd won the battle but not the war.
Since the last post I have replaced the HT rectifier (more on that in a second), but I first replaced C93 (0.05uF/350V wax/paper type) with a 0.047uF (473J) 630V polyprop type. This resulted in the raster being much compressed, as in the image below. The image below is of the raster as its maximum now.
I have checked my work around there and all seems OK, so I thought that if the HT and everything else was OK, then the presence of the old C93 might have meant the picture may have been much higher than it should have been.
After this the height pot did have an effect, but mot much, the total effect between max and min height was only about an increase/decrease in height by about an inch or two.
After this I replaced the HT rectifier with a 1N5408 (leaving the 15 Ohm surge limiter that checked out OK alone) and this made a huge difference to the HT, brought it up to about 210V, but still short by about 20-27V. So I inspected my work on the mains selector (remember, the mains selector knob had fallen to pieces, so I soldered in a link behind) and discovered I had set this to the wrong voltage. Resetting to 240/250VAC and cleaning the HT fuse and associated connector brought the HT up to a much more respectful 245V - a little higher than the service manual states, but surely not enough to matter?
But the effect on screen remained stubbornly the same. Not enough width and certainly not enough height, in fact no different at all to how it was before I had changed the HT rectifier and sorted out the mains selector.
So I fished around and found R108 (1M) had increased to 15M (tested by lifting one leg), and then found that it seemed to be acting as a thermistor, for about 2 minutes after the set was off, the resistance decreased back to about 1.7M. Solder back in and switch the set on for a few minutes, then switch off, desolder both ends this time and measured and it repeated the process.
So I replaced this with a 1.2M (all I had in stock) and this seems to be staying stable, but when checking around the height pot, I found that there is a dead short between both ends of the pot, whereas the maximum the wiper can get up to is about 300K. I began checking around the height pot because after I had replaced the R108, the height pot then had no effect at all on the circuit, the height remained fixed at what you see in the below image.
As it was getting late I forgot to take the boost HT voltage, but I guess if the height pot is short, that would be bringing down the boost HT, possibly resulting in a lack of width.