"There has been a slight design change to these since I last bought one; the mounting has four slots rather than three screw holes, which can be a bit of a fiddle to line up if you are drilling into masonry. In case you take the thing apart or aren't sure about the order of assembly; (1) Push an insert (sold separately) into the cut end of the pipe. (2) Slide the blue nut onto the pipe, then the white split bush - flange towards the end of the pipe. (3) The black rubber seal should stay inside the fitting (bevel edge facing outwards), and the black flanged ring should just rest on top of it - if these are too tight or misaligned you won't get the pipe in. (4) Push the pipe all the way into the fitting - it need to go right through the rubber seal and hit bottom. (5) Slide the white split bush all the way up to the black flange, then tighten the blue nut onto it by hand. Hard.
If you get all of that right the white split bush will tighten around the pipe so that the fitting can't rotate, and the rubber seal is pinched by the black flanged ring, to form a watertight seal (a bit like an olive in a compression fitting)."