I've seen mention somewhere that leaving GRP bodywork out in the sun can help make it a bit more pliable to help with fitting. Will I get the same effect bringing the panels inside to a hot room (30c) for a few hours?

I doubt it, I found only high temps worked for me.

With my black bonnet in the sun around summer time it would make it more pliable, it was almost too hot to touch. not at 30c

If its very thin you might get some help from a fan heater.