flooding

Someone we know had the same problem before and has put in a non return valve and problem solved

That seems very weird. Are you sure there is no vent going through your roof. As most buildings in the UK have a 4inch cast iron / plastic breather. When there is heavy rains that is what usually happens to most places. There to much water volume for a 4 inch pipe so it finds its way out.
The sewers here are very 1900s. I don't a lot of work in battersea where house where on street level with the main sewers. What you would find is that as soon as the rain got to much for the system it would back up at the lowest point. I've installed many flood gates on interceptors ( the point before it leaves for the main sewer ). They are a very good flood protection. All it is is a flap screwed on to the side of the benching in the man hole and when there a flood the water closes the flap and prevents it flooding your house.
Sorry or the long discription.

thanks for the responce i like the idea of the flood gate , will have to investage this more thanks