Sarah faces one of the most shocking properties yet where mysterious leaks have turned Denis Richardson's home into a swamp. A permanent smell of sewage lingers in Denis Richardson's basement flat in London's Gipsy Hill and the home is a health hazard of faulty wiring, collapsed walls, and running waste water. If that wasn't enough the garden is being taken over by Japanese knotweed. A damp specialist describes the leaks as the worst case he has come across in 20 years. As they prepare to tank the basement they also discover that at some stage all of the internal load bearing walls have been removed, leaving some of the shared walls without foundations.

• Sarah Beeny