[Diabolical Artificer]

Thanks Dave, I knocked the circuit board up yesterday and will do some proper testing, trying to test on breadboard with 100's of volts about isn't optimum.

Regarding the MJE350 CCS I looked into and tried a few circuits, namely a Sziklai pair CCS, and a Darlington CCS, the first baffled me and I couldn't get it to work, the second worked ok but I only had a 100v PNP smaller better beta Q, volage drop was equal between it and the MJE350. In the end I've gone for a gurt big heatsink mounted off board, Q sandwiched between chassis and said HS.

Looking at several datasheet's for high voltage Q's all show about 10mA max at about 300v for all Q's whether they were rated at 300v Vce or 1000v Vce, this even though your not exceeding the max power rating. For instance the MJE350 is rated at 20w @ 25 deg C, running at 270v Vce x 0.010A is 2.7W, however it was definately hotter than 25 deg. That said I've used them and the MJE340 at around the same level without issue, perhaps I'm worrying too much.

As things stand I've used my potential divider with mosfet "follower" to drop 100v ish, will see how it stands up today. more later, Andy.