Okay, to answer Robin's questions first...
(1) We know there is spark because you've run it with a loose plug in number 1 lead?
- yes number 1 and number 4 have both been out and are both sparking
(2) We know there is fuel circulating because you've removed the return side and run it through a long hose into a bucket or can?
- yes if you mean disconnecting the fuel line at the upstream end of the fuel rail and getting a mate to hold it while the fuel is fired all over the carpark 
Eventually started using a fuel can after getting worried about standing in the puddle. Repeated this a few times over the last couple of weeks and have pumped about 2litres in total through the system. When the pump primes a squirt of fuel is seen as expected. Then when cranking there is a constant stream of fuel under good pressure as long as the key is turned. The system seems fine...
If that's OK, run a longer test - keep cranking (you can remove spark plugs to reduce load on starter motor) and see whether the fuel return dries up after a while. This could be the case if you had a corroded connector in the engine harness connector blocks en route to the fuel pump. When cold it might allow enough current to prime the motor, but on continuous running the connector will heat up (there's a lot of current flowing and even a 1 ohm resistance will generate significant heat) and this heat causes the resistance to increase - vicious circle time until too much voltage is lost across the bad connection and the pump stalls. Leave it for a while to cool down and it starts to work again
. - so what you're suggesting is, in this case the car should start but after getting hot, will die. This isn't happening here - its not catching at all
So if on average it runs for 20 seconds after catching, try cranking for a 30 seconds and make sure fuel is returned at full flow for that whole time.
- it won't catch at all if the fuel line is connected, it only runs (and runs quite sweetly) when the fuel rail is at atmospheric pressure and of course dies when the fuel in the rail runs out. Throughout this period, as mentioned, the fuel pump is quietly emptying my tank into a fuel can...i.e not returning to the tank
(3) I assume it's not now backfiring (i.e. there's no way you've fixed the fuelling problem and introduced a firing order problem in the process)
- absolutely no change. Once the fuel line is reconnected following the engine running ok without it, the engine just turns over without firing
(4) Although it's a no-no in normal running, have you tried to start the car on part throttle? I assume it will just splutter and die as before. I would also disconnect the TPS wiring and run it without a TPS (the ECU doesn't need the TPS to work and will just ignore it if not present). Opening the throttle manually will bypass any problem with the IACV not opening.
- no effect using either the throttle or opening manually. Not even a hint of a fire
(5) Given it's now quite cold if the problem is that the coolant temperature sensor or inlet air temperature sensors are gubbed, it could make a significant difference. Disconnect the brown sensor at the water elbow and the green one on the intake manifold.
- have looked at at least one maybe both but surely this wouldn't let the engine run when the fuel isn't pressurised?Oh, and it was warm when the car cr4pped out
(6) Now that you have a spare fuel rail you could consider running your original fuel rail loose into a bed of paper towels and see just how much fuel gets squirted into it when cranking. Note that the ECU doesn't do fuelling in an obvious way on cranking - rather it just squirts now and then to avoid flooding the engine, so provided you get some fuel into the towels, I think you can be happy.
- this is interesting. I'll have a looksee again this weekend but I think the fuel is coming out a fairly constant rate when cranking. Flooding the engine right from the word go? Only thing is I would expect the exhaust to be stinking of fuel and the plugs to be soaking when pulled - neither of these is happening hence why I thought under fuelling rather than over...
Dan/Richard - with regards the Compression Test, surely if the engine was running (regardless of the fuel pressure) then the cylinders are having a poor-boy compression test each time it turns over
Craig - thanks for the offer to lend your ECU. But......to my mind, the thing that is preventing firing must be related to the fuel pressure in the rail (works when atmospheric, doesn't when its not) and to the best of my knowledge there is no electronic sensor recording this so the ECU doesn't need to know this before firing the plugs 
I've now got the new fuel rail and new FPR and I'll get them on in place of the used parts (I'll also check the return flow in the process). I've just ordered a new coil as well (cheap enough to easily justify) to completely rule out the ignition system.
I have to get focussed as my old man wants his Vectra back! 
