My HC1100 is leaking from the drain plug in the bottom of the tank.When i try to tighten up the screw it just goes round and round. The plastic bush which the screw goes into also goes round and round.There is a small drip of water coming out of the drain plug.There are no spare parts available to cure this and i have been told on the phone i have to buy a new boiler.Any advice on how to solve my problem would be much appreciated.