Hi OK I have fixed it. Many thanks for info above but it was not one of them. The fault was the pressure valve on the right hand side of the overdrive unit, I just took out the spring and small shaft also the ball bearing. It was all cleaned thoroughly and replaced, took it out for a test run and all was fine. I think it might have been a a small blockage or the valve had got stuck in the off position. Whatever it was all is now working.