I think there must be something wrong with the soil if this is happening. Do the drains run under there? I think Honey fungus smells more of mushrooms than sewerage, but it is a possibility.

Environmental health if you suspect anything to do with sewage leakage, but the thing that springs to mind is honey fungus. You may, if you dig around in the soil around affected plants, find black "bootlaces" which are typical of honey fungus; alternatively, peel back the bark if you can on affected plants to see if there's a white growth inside with a mushroomy smell. Otherwise, the toadstools associated with this condition usually appear in late summer or early autumn and are, as their name suggests, honey coloured. If it is that, there is no cure, but you will need to remove all and any dead wood anywhere in the garden to try to halt its spread, and then start growing more honey fungus resistant plants.