Well, the latest in the saga is that the old girl is now up and running (long may it last)!!
Managed to get her running and get her to a local diesel injection specialist. The plan was to check the injector coding but they could not access the PCU. They put me on to another local garage who had the correct Ford software and we hooked her up. Seems that there were no codes at all in the ECU for any of the 4 injectors. We programmed the codes in and we were able to clear the fault - no more flashing light - return of the turbo!!
Has been various stories with re-coding on web- it has worked for some people but not for others so lets just hope I fall in to the first group!!
So to recap......new fuel filter (£20), knock sensor (£25), injectors removed and nozzles put in ultrasonic bath to remove build up (abused company facilities) and finally, recoded all injectors (£60).
Good luck to anyone who has the same fault!!