[Fordie]

Depending on what type of waste you have, some waste pipes go out through the wall and into a back inlet gulley,this is another form of trap ,these can block and stop the flow of water causing a back pressure. Another one can be bends in the waste pipe the first couple of bends may be clear but if the last bend in the system is partially blocked again this will cause a back pressure . If it cannot be rodded, get a compression coupling ( Macalpine) cut into the pipe and inspect it ,clear it, then reconnect with the coupling. 4D