can anyone recommend some good, reasonable hostels, or b and b in cornwall
Have not stayed in any of them but here is the England and Wales Youth (any age!) Hostel Association site. There are reviews. http://www.yha.org.uk/find-accommodation/south-west-england/index.aspx
For the north coast, Headlands Hotel, on the cliff edge near Port Isaac and Tintagel. For the south coast, Housel Bay Hotel, on the Lizard, also cliff edge.