This smell only comes when the hot water is run for the bath. It is most evident in a separate compartment containing the combi boiler. The plans indicate a Durgo valve, but I have been into the attic where the SVP goes up, but cannot find it, here or anywhere else. But it is clear that the smell is not coming from the bath or basin drains, or the toilet. And why should it be associated with running a lot of hot water? (The combi, eg for central heating, does not do it without running hot water).