Hi Calum,
Dale's pretty much spot on about the code. You're working from an existing code base, but can do with it what you want with it. Part of the challenge is to work from the existing code though, so try to make clear any changes that you make to the original.
As per the brief, the release platform is Windows PC. You're welcome to expand the development to other platforms, but will need to have the desktop version as your submitted final project.