Thanks everyone, I have the engine running, but I'm not sure I really understand what the problem was!
I loosened the big nuts on the injectors and cranked, and lo and behold drips of diesel started leaking. I removed the air filter to expose the metal air intake pipe, and tried again (in case it was some kind of blockage) but that made no difference. For future reference, when you say a blowlamp in the air intake, do you mean blow a flame into it with a blowlamp? Directly in or just near, to warm the air?
I just connected a thick positive cable from the "domestic" side of the split charge relay to the other "starter" side, essentially bypassing the split charge relay which usually separates the two positives. When I measured the voltage of the starter battery at its terminals, it was only 6V. Then I realised that there was a battery isolator switch that appears to isolate the starter battery by disconnecting the negative linkage between them (not sure the point of this). This isolator was off! When I turned the isolator on, the voltage showed my domestic battery voltage. So clearly the problem was I was trying to turn over the engine at 6V!! I'm amazed it did anything at all really.
So the problem seems to be a dead starter battery. I might just get rid of it and use the domestics only. The make is supposedly a "hybrid" that is happy to act as a starter battery.
Once I solved the voltage issue and I was getting a proper crank out of the starter, the engine would crank and every now and then give a little cough as if it was about to start. Lots of white smoke. It took about 3 or 4 attempts of 30second cranking (the longest I'd dared to do) before it came to life. When it did, the note was a bit more of a deeper grumble rumble than I'm used to.
I'm letting the engine run for 30 mins or so (at idle and with the alternator disconnected so as not to apply load) to get it up to full temperature as suggested.
I fear that my once bombproof beta might not start as easily as it used to anymore though