Hybrid design done .....
First - thanks to all who reponded.
Second - Oznog - Excellent suggestion, but this sort of thing is not easy to find on the second-hand market here, and besides, despite my caution, I really want to try and build this thing myself.
The hardware aspects can, with a bit of ingenuity, be solved. Not, perhaps to high-class professional standards, but certainly good enough for my home studio.
I have cobbled together a circuit, using sections from a couple of posted designs, and included a switched-level incandescent lamp dimmer so that I can use modelling lights before triggering the flash.
I'll post the schematic before attempting constuction and would be grateful for any comments.
One lat question: When the flash is triggered, are the modelling lights switched off? I would imagine not, as the light level from the flash will certainly over-ride the incanscents, but will the incandescents cause a colour temperature shift, or give a slight red cast to the exposure?