We just got back from two weeks in the Truro/St Austell area, and we hired a car from Blackbird Car Hire, based in Bodmin - they specialize in meeting you wherever you are (like the train station) with the car, and then they pick it up wherever you want to drop it off when you're done. They are a small firm and very friendly. Also they have a wonderful feature which is that they don't care about small dents, scratches, etc - our car had a few cosmetic blemishes, and they said as long as we brought it back reasonably intact, it was ok with them. (What a difference from using a big firm out of Heathrow last year!) I would definitely recommend then.