It's a high-voltage equivalent of an LM317. Although not an ideal solution due to power dissipation, it will replace the Darlington and the LM317 in your circuit.
You'll need a good-sized heat sink for the regulator. If your load is 20mA, the regulator will dissipate around 2W of power. Copper heat sinks are much better than aluminum, as they conduct heat much more efficiently. If you are going to use a fan-cooled heatsink, use one powered from the genset output, not from the output of the regulator.
[eta]
Oops - won't work due to minimum 14v dropout. 12v - 14v = nothing out when trying to start up.
All good stuff folks thanks.
I will look at both options and see what I can actually buy without international shipping.
I didnt even know High voltage regulators like that existed !!!
It is also clear that I need to optimise the rest of the circuit to minimise the current consumption and hence power disipation of the power supply, whatever that ends up being.
I have posted my curent progress and issues on my origional thread, so the full picture and asociated issues are all in one place.
I think that will be better from here on in.