Plastruct do an enormous range of tubing, elbows, T's, flanges reducers etc. but, though some of the range is readily available in the U.K., most appears not to be (well, not that I can find it). Â I've ordered direct from Plastruct in California without any problems; here's their website
http://www.plastruct.com/
However, it can work out VERY expensive, though each part isn't too much, once you've had six of these, half a dozen of those and three of them, it mounts up!!
Arthur