Colc, bit of a long shot, with with regards to the the route from your tap hole into the ground. Is there a U-Bend in the middle?
Or is it pretty much straight down into the ground
We had a problem a few years back where the outflow from our washing machine essentially went straight down pretty much (there was a bit of a dog elbow type turn but not enough to stop some nasty smells emanating. ...... my understanding is that an actual U-Bend catches gasses that rise up from your drain and prevent them going any further...ie into the house).
Otherwise if there IS a U-Bend (which to be honest i'd expect there is), have you checked it out ?? (it could be full of all sorts of unpleasant bits and pieces....dirty job but doesn't take long).