[Tyso_Bl]

Thanks for the suggestions, I've studied the circuitry round this rectifier, and it purpose seems to be to prevent priming current leakage from the trigger of a cold cathode valve (fed via a 200 Meg resistor), in to a voltage divider network on the other side, while still allowing the voltage divider to influence the trigger voltage. The current is in the microamp region, any back leakage would totally negate the purpose of the rectifier / diode.

Slightly aside I've measured the capacitors in the unit as being leaky too, showing 15Meg for the worst ones. 0.1uF 350V.

On the verge of putting it away for another day.