But then shouldn't the bottom end of the vertical bit be threaded? It's not, it's just a piece of pipe, so you can have the big nut as tight as you like and the pipe still just pops in and out. And the copper pipe is green directly underneath the u-bend, so there seems to have been a leak here...
We've just replaced our washing machine and in order to get a straight connection to the cold water pipe I had to temporarily remove part of the drainage pipework, as it was in the way. When I connected it all up again I had a leak and I *think* it's because it had been bodged before by the...