leave the tap running with the boiler turned off, most likely air in the pipe work, if that does not work bleed your pump and possibly the radiators. did you drain your system.
is your air vent open
I'll try running the tap with the boiler off now. No air in there. I only drained the bolier and not the radiators. The manual said the I didn't need to drain the wholw system. I thought the hot water and central heating ran independantly from each other? i.e if the radiators are low on pressure the hot water still worked.