Nok

I'm new to this site and am having difficulty navigating it. My original post does not appear: I have an existing 11/2'' plastic waste pipe with a 12m straight run and the return legs are connected to a sink, the other into a manhole. There are three shower wastes & traps connected to the long length. I am frequently suffering from bad smells, I'm told that there is virtually no fall in the pipe length and wastewater accumulates. The pipe is located within a 10mm void between the structural wall and stud partition. It is impracticable to get access to replace the pipe run. I'm told that the pipe diameter is too small to be jet-hosed, I regularly pour diluted caustic soda or hydrogen peroxide down the sink. I feel that the smells may occur by back siphonage of the shower wastes. Any suggestions to rectify this would be appreciated

Thai massage spa. I've attached a pipe layout

Hi I'm Nok, I have a shop, in the NW, that was fitted out by a cowboy and am frequently having problems.