darn, I'm 28th Sept to 4th Oct, based in perranporth, and mainly doing South and Central (already spent a lot of time on Bodmin Moor), plus a quick day trip to St.Mary's. Boat is very cheap for a day trip - £30, but takes a while and only gives you about 4 hours on the island.
All you need to know about boat, and skybus (plane) is at: www.ios-travel.co.uk
Ditto re chopper is at: www.scillyhelicopter.co.uk