First of all sorry for taking my eye off the ball on this but other issues have taken fore stage in life I am afraid.
Rob; sorry for not replying, I made the error of saying to myself "lets see how we go, I will know in a few days"; then life sort of fell apart again!
Lucien; I have plotted the voltage across the regulator's coil with respect to panel input voltage (from a variac - before I refitted it) to see if my hunch that the Metrosil units provided an offset so the coil was only energise over the upper part of the voltage range. this appears to be the case. I also had a look at the resisitance of the pile over the controlling range as well. Here goes:-
(machine o/p volts - coil volts and where given pile resistance)
70v - 1v
100v - 7v
125v - 11v
150v - 14v
175v - 21v
200v - 26v
210v - 27v
215v - 29v (coil starts to buzz)
220v - 33v - 9.1ohms
230v - 35v - 10.3 ohms
240v - 40v - 11.6 ohms
250v - 42v - 18 ohms (but wanders a bit)
I noted that someone has scrawled on the exciter terminal panel "13.5vDC for 250v".
Regards Ian