This post is for the benefit of anyone else experiencing a problem whereby their TDCi will crank but not fire, but fire up OK the second time. Lots of other people seem to be having the problem over the last few years, myself included.
Anyone doing research on Google may be interested to know that I had the starter motor and crank shaft position sensor replaced, but this did not cure the problem. However last week they finally found two fault codes in the system. One was relating to the Fuel Rail Pressure Sensor, and the other to the Injectors. They cleaned the connections on the sensor, as they reckoned it was a connection problem rather than failure of the sensor. For the injectors, they reprogrammed the injector ID codes back into the system, as it seemed to have lost them.
Having driven the car for a week now it does seem to start more confidently and is certainly a lot more perky than it's been for a while. Won't know if this has definitely cured it for another month or so, but thought this info might be of use to someone.
Gary.