Been out to a job today and both sinks have a very slow draining blockage. Its was a double sink, when I ran the tap in one the other filled up. So I stripped down the waste pipework underneath. nothing unusual. got rid of a load of waste material. nothing substantial to block drain. tried again and sink doing the same thing. I then thought it was outside. Went into conservatory where the waste come out. I am met with a swept Tee facing the correct way up with an access cap on (solvent welded in). I remove the screwed access cap and the water emptied from the sink straight away. Am I right in assuming I need to get a anti syphon and fit it to the top of the Tee? Issues I think I have: The person who fitted it did not sleeve the waste through the wall so I cant move it because they have cemented the tee in the wall. also if im right about the syphon, how do I remove the solvent welded access cap to fit the syphon or can I get a screw on one?. oh its 40mm pipe.