Hi Peter, David and Joe. With your help, I have fixed my car, so thank you all. Before getting into what I did though, I would like to clarify the thing about a potential £300.00 charge for for diagnosis-rather than reading the fault codes, which my mechanic had already done for free, that was for a full system check-injector spray patterns, pressure at different points in the system, software analysis, fuel sample chemically tested etc, all using Delphi specific kit. Anyway, in the end I didn't much fancy that. Based on your advice and other threads on here, I decided to change the cam sensor, air and fuel filters, fit an EGR blanking plate and run some good quality fuel cleaner through it. In the process of doing this I also discovered that a vacuum tube was not fitted to the underside of the air box, the inlet snorkels were not fitted, the pipe between the turbo and MAF was split and an electrical connector in the engine bay was burnt out (not sure what it is, a fairly large wire to a stud, where another wire picks it up, running along the front of the engine). All in all, I'm surprised the old boy ran at all. I fitted the new parts and repaired all the faults, and cleaned various other electrical connectors....and my car now goes better than it ever did, it runs like a sewing machine and pulls like a train and is a joy to drive. Thanks again!
BTW, when replacing the fuel filter, I remembered an old tip a mechanic once gave me-turn the new filter upside down, unscrew the drain cap and carefully fill it with neat injector cleaner. This primes the system (Worked at treat, it fired immediately) and gives the injectors a good dose.