My first port of call would be Griggs farm supplies in Hewaswater just outside Snozzle, tel:01726 75371 Duchy Timber in Lostwithiel although they can be a bit pricey.
Thanks gents. I am starting a job down there in a couple of weeks and would rather buy my materials locally and from recommended suppliers. At the moment I am waiting on a price for 30 posts @ 200mm x3m round posts delivered from wales, seems crazy
200mm 3mtr round pole ain't going to be cheap, try a agricultural merchant like Mole Valley Farmers, they got branch in Cambourne & several other branches in cornwall. Could try Mike Hawkens at St Columb ,another is Walter Bailey at Par.
Timberstore on ebay with branches in Devon and St Austell Cornwall. http://www.ebay.co.uk/itm/8-200MM-D...OR-/222544354295?_trksid=p2385738.m2548.l4275