eracode: The coefficient of thermal expansion of Marley PVC-U is 0.05mm/metre/°C.
So an eight metre run coping with, say, a 25°C temperature variation, will produce a change of 10mm.
I found the above coefficient by googling and the hit mentioned Marley by name.
Marley’s own site says:
“ ... uPVC has a relatively high rate of [thermal] change. The coefficient of thermal expansion for PVC is 7 x 10-5/°C. A handy rule is 7 mm change in length for every 10 metres for every 10°C change in temperature.”