Thanks for all the input. I have to say that the fact that you can get full power ok whilst is reasonably cold suggest a fault like is suggested. Key thing was the car was allowed to get hot before the dyno run & we already know it plays up when hot so that could explain what Jamie found.
Think I'll hunt out a dispak from the breakers as the next least cost step to eliminate possible faults. I can let it get nice and hot and then rag it down the drive. I'll soon know