Woah woah, it seems its all kicked off here while ive been tinkering.
I have a lot of experience with cars (and motorbikes i might add, im a design engineer at Triumph Motorcycles in Leicestershire). I DID crack open a line to one of the injectors. I did this knowing it would be safe as ive had plenty of experience with high pressure circuits having owned hydraulically suspended Citroens nearly all my driving life. High pressure doesnt mean panic and fear and big blasts of fluid all over the place with a leak. Quite bizzarrely Citroen dealerships refuse to work on the 'high pressure' elements of my BX because they're, to quote them: 'too dangerous and need specialist attention'. I always thought a dealer WAS the specialists but it seems they dont trust their own mechanics any more than i do
The actual volume of fluid moved is relatively low, so if you open the pipe unions enough, all you get is a steady trickle. IF you start tightening it up whilst still cranking, you turn that trickle into a mist of diesel but its not really that scary. I've been working on this on my own, so the car couldnt be cranked at the same time as tightening a union so i undid it, then stuck a rag around it and went to crank the engine over. Stop cranking, go back to rag and see if it is heavily soaked - diesel is flowing. I WOULD recommend this as a way of determining whether you have diesel getting to you injectors - i did it, and i survived to tell the tale with no damage to myself or the car...or the rag
On a much more chirpy note, the car is fixed! Don't ask me how though. I borrowed a Bosch handheld diagnostics unit of a mate and it flagged up about 6 error codes from previous faults. I cleared these, unplugged it, turned the ignition off turned it back on and no more codes appeared, i then tried to start the car and guess what....it fired up first time.
This is good for me because i now have a working car, but unfortunately it doesnt really give a definitive answer to what the problem was in the first place. Is it possible that the cold weather threw out a code temporarily and once theres, say 6 codes, the ecu decides its just not going to let the car start? I get VDC and motor control unit warnings flashing on and off all the time (preious to this non starting issue) and i've read this is 'normal' so i can only guess this has triggered some sort of total shut down after repeated loss of communication with sensors and wipingthe errors has allowed the engine to start.
Thanks for all the suggestions and advice along the way, much appreciated!