Large Farnell PSU trouble shooting question

[oldway]

The other thing I have at the back of my mind is that I am driving a powerful 2kW low frequency RF amp (actually two 1kW amps into a combiner), so there's lots of 136kHz RF about.

Each amplifier has its own low ESR 4700uF across the supply with a paralleled 0.1uF ceramic as well.

Ok, it is different of what you wrote before...

What's the best way to check the pass transistors are being kept in a sensible voltage drop range by the SCR's please?

With a multimeter measuring the voltage emitter -collector of a pass transistor with high load and with output in short circuit and max. current.

I did not look at the technical informations, but you should find the description of the calibration and adjust procedure.