loss of pressure?

I have a worcester 28 cdi combi boiler, if the bolier is left cold the pressure holds ok. If i put the heating on for 2 to 3 hours, then turn if off and leave the boiler cool down, the water pressure drops to zero. I cannot find any leak anywhere on the heating system, or the boiler. Can anyone please suggest what the problem might be

Ian
November 2009