Sorry John, The 30P4 was NOS I didn't expect it to fail so quickly.
Anyhow, I've finished checking the circuit. Please refer the circuit attached, it's not the same as the Murphy Service Manual or Trader sheet.
I had mistakenly fitted C312 to the wiper of the line hold pot and not the screen grid of the 30P4.
R335 was being pushed by the Line Linearity slider against a chassis screw and it had completely worn through the resistor's body effectively making it a 500R resistor instead of 1K. I was also in the area where I had previously heard arching
.
I've also borrowed a valve tester and the 30P12 and both 6/30L2's were very low emission.
Do any of the the above faults explain my collapsing picture and/or wrecked 30P4?
Regards
David