Prizes this time go to Bolds and Nelly for correct diagnosis. It definitely seems that a new crank sensor has cured it. WHOOPEEEE!!!
Many thanks to both of you.
Chris

Oh dear. Used it three times to get to work on. Tried starting it to go home on third day and it refused to start. Then the dreaded light came on again!! This time the fault codes showed 12.2 and 13.2, fuel injectors shorted to earth. Also the fuel pump whirring noise had stopped. These symptoms last appeared 10 months ago when the fuel pump relay gave up the ghost. A new one cured it that time so I've ordered another. The thing is, do they tend to break that quickly or could it be something more sinister and difficult to find? The new Gixxer 750's are beginning to look quite nice- and reliable. Any ideas anyone?
Chris

I would say buy the lack of replies it should last longer than that,
I think you got a friday afternoon special,