Crescent House, Smell of sewage in bathrooms

[Ben Langlands]

Hi, Sorry about the unpleasant subject, but does anyone else have a problem with a strong smell of raw swage entering their flat in Crescent House through the high level vent in the wall of their bathroom? It happens to us most mornings between 6.30 and 11 am and is especially bad when it's raining. If anyone knows the cause, or anything I can do to stop it I would be really grateful. I have fitted an extractor fan with non-return flaps but it doesn't help much.