How can I restore the hot water supply from my Potterton 45 back boiler?

I have a Potteron back boiler for central heating and hot water.Until recently it has all worked OK but now I cannot get hot (only tepid)water with hot water only selected. The pump is OK., the boiler has been serviced, I cannot find a priority valve or a tank stat and the best suggestion I mhave had is to drain the system because it is probably an air lock. I have bled the indirect connection on the tank but no sign of air. The boiler is a Housewarmer 45/55.I can carry out any work required.