I realise you've all got better things to do than worry about my engine problems, but here's an update.
Drained all fuel, cleaned tank, replenished with fresh. Bled at pump, bled at injectors, all flowing freely. Still no start, eevn with start spray.
THought again about hydraulics. Checked fluid level, all good. Found what I think is a pressure control lever (next to the seat, down the right hand side). Moved it all the way forward. Not sure what this does, but did it anyway. Tried starting it, quick spray of starting magic and it jumped into life and ran. However I couldnt keep it running, felt like it was struggling and not particularly happy. Eventually died and couldnt restart. Ran for about 10 seconds.
Is this a step forward, or complete coincidence? Is this lever a pressure control?