Anyway, time to bring this thread to a close as I've managed to put everything back together, replaced the battery without incident and she started on first crank after the glowplugs did their thing.
The AA was spot on with his diagnosis who remarked about the cloud of smoke out the exhaust on starting. Good tips to remember, I think.
Well done to Jim for making the same correct diagnosis.
Thanks everyone for your support 8)
The question now is, should I sell it for a quick profit before I get too attached to it and buy an even better condition xantia 1.9TD? Other options are to put it up for the MOT and see what work is necessary or just carry on tinkering until MOT expires (end of this month) and take it from there?