I went round to a friends house who's also a hobby moulder.  He's been having trouble with his mouldings warping.  They are CSM polyester and have moulded fine.  Over time (up to a year) they are warping really badly.  One example is a panel that has a 90 degree return / lip which has curled up beyond the 90 degree!  I've never had this and all I could come up with was demoulding too early.  Are there any other reasons he could be seeing this?

Nah it won't be anything to do with the gelcoat. It is possibly undercatalysing of the reinforcement laminate prior to demoulding, as this would then take a very long time to cure and explain how long it has taken to distort.