Thanks Artic,
I did a little more investigating and found a couple of things.
There's a LM317T CC12CVW mosfet on the PCB that heats up very quickly. It does have a small heat sink but after 10 seconds use the heat sink becomes too hot to touch.
From the 12v input supply it looks like it feeds the mosfet ,on through to a resistor and diode , to a 25v 100uf cap, to another resister , then another 25v 100uf cap , on to another diode and finally to the high voltage generator.
By the time the 12v input reaches the last diode , it drops to 3.1v
It looks like the hotter the mosfet becomes , the more the voltage drops off .