OK - further update.
I had the second set left running in pieces, as I did the previous one - but unfortunately I've been off work since last Saturday looking after my wife who has a chest infection, and was too ill to left alone. So the set has been running unattended every work day, as it powers up when the workshop power is turned on.
Needless to say. this has burnt-up as well but somewhat worse, as I wasn't there to keep an eye on it, and the board has burnt entirely in half!.
Again I can only presume it's due to one of the tubes going out, and the voltage at the mid-point going to high, and arcing to the chassis.
I've ordered a new PCB for it, they come from Germany, so should be Monday or Tuesday - presumably that will cure it - I've already got a pair of new tubes awaiting fitting.
BTW, the previous set is still running fine, with the back on and all trips reconnected.