After having a flood caused by a faulty ball valve and the loft tank overflow pipe falling out of a bend. I made these pipe clips to hold the overflow pipes in place.

There are two clips one holds the pipes horizontal and one with an offset of about 9mm so It could clamp the overflow from the central heating as well as the hot water header tank.

I don't know the manufacturer or plastic material of the pipes but the measured the outside diameter was 23.47mm. 

Here are the dimensions of the clips: