Final update on this posting! I have now replaced both faulty crash sensors as well as the air bag control unit with second hand 90 day guaranteed units. This was very easy to do. No components needed reprogramming and after the final error messages were deleted the air bag light went out and stayed out. Many thanks to my local Ford dealership who I have to say were incredibly helpful and did not charge me for either reading the error messages or clearing them - this took 3 visits. Final observation is that the air bag control unit is advertised on some used parts sites as an air bag ECU - I think that is misleading as the ECU implies part of the Engine Control Unit which as we all know it is not. Anyway, after an original quote of £600 to £800 it has been done for £98 - MOT next...................... I hope this experience will help someone out there.