Main HE 24 Combi Smells of egg when hot water is run?

My friend has a Main HE 24 Combi and it really smells like rotten eggs, but not gas, when the hot water is run. The boiler is only about 18 months old but he doesnt have the manufacturers instructions or any warranty info. The area is particularly bad for hard water, not sure if this would have any influence. Anybody got any ideas or had a similar problem?