I suspect that you have an air lock in the hot water.
Is you coldwater of mains, if yes then go to the basin and put some hose pipe between the hot and cold, open both up allong with the shower the cold will force its way through the hot pipework and fill it. Make sure however that your header tank does not overflow.
Richard Hall - Let me know how you get on rich@the-halls.co.uk, July 2008