Ok guys thanks for all your comments. It seems then that this behaviour is normal for long runs of pex pipe when running hot water through it. Mine doesn't need to look neat as it will be hidden, but I was worried that all that flexing as it get hot and cools would cause problems over time. It doesn't seem to be happening near the joints at either end, more in middle of the run so hopefully all will be fine. I'll try and allow more slack on the bends (There are two 90 degree bends in the run) to take up the expansion.
Does anyone think that replacing with a different brand of pipe (I was thinking speedfit) would eliminate or at least reduce this effect?

Cookie, it is not necessarily the brand more the type of plastic used, for example ABS expands nearly twice as much as PVC but I don't know what brands use what; on a rough calculation even ABS should not expand more than 12mm over 4mtrs with a temperature rise of say 50C; if you can find the materials used then this is an interesting list:-
http://www.engineeringtoolbox.com/linea ... -d_95.html