I started a second mould (Unimould system) a few days ago, and the gelcoat is not hardening as it did the first time.

Thanks Matt - I'll give it a go! For completeness, I'll post some photos tomorrow.

Is the rubberiness around the edges? It tends to be as this is where oxygen can get to it, from the sides.

Also, is the rubberiness in concave areas? Styrene is heavier than ambient air and will pool in concave areas. Turn the mould upside down for a couple of hours to drain it.

After that I would just carry on and laminate.