My first instinct is to tell you to thrash it since it only cost £39.00 + shipping, but then I realize something like this would go a long way in honing your prop and costume making skills, even if you fail at this one eventually.
Though not as pliable as styrene or ABS, fibreglass CAN be shaped to a limited degree with a heat gun. It takes a lot more heat to make it soft enough to reshape, so you're gonna need serious protection on your hands to work it. The fumes coming off due to the heat is also very toxic, so make sure you have a good respirator on, an work in an area with good air flow and without having other people around.
Work on a small area at a time, and don't force it or else you risk cracking the fibreglass. If it won't bend heat it some more.
Rinsing /dumping the reshaped part in water help it set quickly, and lessen the amount of time you need to hold a scorching hot object.
Also try to grind down any edges/seams that don't fit properly so it becomes easier to bend/shape. You can reinforce it later with epoxy or bondo once you get it to fit properly.
That's all the advise I can give without knowing what exactly are the problem areas. If you ost pics of specific areas you need help on maybe others more experienced then me can chime in and give their 2 cents.
Happy building!
