Eww, Corsas. Anyway, for £78 you are getting burned. Find a local mechanic, buy him a pint and be his friend, generally they don't charge you just for looking in that case.
More than likely it's a screw loose. Although are you 100% sure it's inside the door? I had a really annoying rattle around that area (it's hard to tell from the drivers side), turned out to be something important.