Good news! I finally got my car back from a different bodyshop, and the decal's come off without damaging the paint underneath. They said it was difficult to get off, though. Cost me less than £50, so I'm happy.
Thanks everyone for your replies which let me know it really was as bad as I thought it was! And thank you Thom for all your advice. I'm afraid it's an unfinished GTi now, but for the moment I can't face putting another decal on it.