Worcester 24i Domestic Hot Water Problem?

Builders have added a radiator and resited another and a couple of days after the work was carried out the Boiler started banging and you felt it would come off the wall any minute. This then settled down and the central heating side of the Boiler is working fine but the domestic hot water side has given up. The water flow from any tap is low and cold. The builders plumber says a new boiler is required as it is over seven years old, but I cant believe that with the Heating side working well.
Regards Keith Ray