Sorry, long title - didn't know how best to describe it.
Much to my amazement, having never thought about it before, I have realised that the end of the A8 third piece of black rubber tubing is in fact a screw-in cap.
The reason that I've just realised this interesting fact if that mine's just fallen out.
Does anyone know where I can get a spare (if available, this may be the smallest spare part on the parts list!).