dreadful smell from macerator toilet

There is a closed thread going back to 2014 about the same topic ('dreadful smell from macerator toilet') but this is a bit different as I don't have a macerator toilet and don't understand how the smell from next door where there is a macerator, maybe more than one, is getting into my home.



The problem started in October 2017, late one evening (sounds like the opening line of a novel!).


To begin with the smell was like rancid meat or rancid fat being heated up. The smell was then only in the back bedroom of the house. It was a really gut-wrenching smell and the bedroom window had to be left open all night to 'dilute' it.



Since then the smell has changed - it's an overwhelming smell of excrement. It basically smells like slurry, and now it's present in the back bedroom of the house, and in the back living room which is immediately below the bedroom. Sometimes a fatty/greasy smell is also present.


It's an intermittent smell during the day, but more sustained and stronger late at night, and into the early hours of the morning. It's so strong at night it will wake me up and I have to either open the bedroom window, or if it's too cold to do that, to move to another room to sleep.



The smell is coming from the next door house who I know have a macerator toilet, in the basement, and possibly in the loft space as well, but I don't understand how the smell from it is getting into my house. There may also be continuing problems with their mains drain following a blockage to their manhole in 2015.



The two houses are semi-detached 3 storey properties. Soil stacks are external and on the gable wall of each house (main bathroom and toilet for both houses are also situated there). We do not share any drains and have separate manholes.



There is a cavity wall at the back of both houses and someone has suggested that the macerators may be being vented into the cavity and into the roof space, although this is apparently prohoibited for health and safety reasons?


There was a plaster and lath ceiling in my basement but this was taken down several years ago to improve ventilation to sub floor timbers. I think this means the cavity is open in my basement?



The next door house is regularly empty for 1 or more days each week and on those days the smell stops.


Next door was also un-occupied between Christmas and New Year and there was no smell during this period. It's owned and occupied by 2 adults, 3 children, and a variety of animals. I've noticed that their bathroom window is never fully closed even in very cold weather.



In 2015 the manhole for next door was blocked after some road repairs and their basement flooded by sewer water. They must have had problems with their drains after this and subsequently had a drainage company carry out some repair work in 2017. Whether this work was successful or not I don't know, but the company concerned have poor online reviews, and forced entry onto my property when I was out, whilst doing work for next door.


Next door are not 'approachable' so can't speak to them about the smell. Am finding it increasingly difficult to cope with. Has definitely got worse since it first started, plus I came down with flu at Christmas and recovery is slow.


Does anyone know how the smell is migrating into my property and if there is anything I can do to stop it?



Many thanks