Hi Dom,
If the boosted side of the cap is 235V that's just the HT being passed through the PY83 diode. So the PY83 is likely to be OK. If the PL81 is glowing, it's conducting heavily and as the screen voltage is reasonable, that current is flowing through the anode via the primary of the transformer. As you can't draw a spark from the top cap of the PL81, it would seem like the LOPT is being heavily damped. Sadly, this is most likely to be due to shorted turns either in the primary or eht overwind. Sorry about that
It's still possible that the boost or other caps are faulty- I would change all the ones I mentioned in the above post if you haven't already done so. Also try replacing the 4k7 screen resistor, although with 180V, I doubt if it will make much difference.
Is it possible to disconnect the EHT overwind where it joins the tag for the top cap of the PL81? If the overwind is faulty, this can remove much of the damping and allow a spark to be drawn from the top cap of the PL81.
Try changing the caps first though and also measure the anode volts of the PCF80 line osc. Do you have a 'scope? If so (as John has already said) it would be useful to measure the drive waveform on the PL81 grid.
I really hope it's not the transformer but I have to say, its not looking too good!
All the best
Nick