In reply to nickhelm2013:
I have been on the Corsican GR20 week that adventure peaks set up, and I have to say, it was utterly fantastic. Rather than provide their own guide, they leased with a local company.
Although this made them, perhaps, a slightly unnecessary middle man, it meant that the guide had good local experience, and that we were contributing to the local economy.
One thing that I would be wary of is the kit lists. They give a kit list, but this isn't exhaustive by any means, and in our case was for the wrong season entirely, so was nigh on useless, but with some common sense this shouldn't be a problem.
I hope that's of some help?