Try contacting the Truro HM, his jurisdiction reaches further than you might expect. Also, try Trevissome House on 01326 374605 (they have moorings opposite the marina and are very cheap - friend keeps his 18 footer afloat there for a couple of hundred pounds per year). Finally, you could join the Truro boat Owners Association for about £10 per year. (Membership Sec on 01872 241364). They have moorings off Mylor and Loe Beach at very good rates.
Regulars will hate me for publicising it but Pasco's Boatyard, St. Just in Roseland, is delightful. They have deep-water swinging moorings but if these are all taken they should have a drying mooring or two available. There is parking and room for tenders etc. Costs last season were £15.50 per foot.
I've had a Shrimper there for 2 years and love it! Only snag may be whether they have room on shore for your boat in winter.
Man to contact is Julian Davey. Number should be in directory enquiries under Pasco's Boatyard, St. Mawes.