Hayle is on the Main Line London to Paddington with a pretty good service. We're told that the sleeper service which leaves Paddington about 11pm is particularly fabulous http://www.firstgreatwestern.co.uk/Content.aspx?id=1555
The ordinary service is pretty good (and can be used for one leg with a sleeper ticket) - as well as the main Paddington Service from First Great Western (You can now buy tickets for selected trains to and from Cornwall until 6th September. London to Newquay from £14.50 - see http://www.firstgreatwestern.co.uk/Content.aspx?id=3826) there are also Cross Country direct trains, or it is possible to change at Bristol.