train from London to/from Cornwall

The closest train station to Tintagel in in Bodmin, which is 30 minutes away. You will have to take a taxi from Bodmin, there isn't any bus service. Edit: Tim is correct you can take those buses, it would take an hour on the bus, buses doesn't run very often.
So you could do either go to Bodmin then taxi to Tintagel a 30 minute trip or go to Bodmin Parkway then take buses to Tintagel close to an hr trip.

There are direct trains from London Paddington to Bodmin Parkway. The standard anytime single fare is over £100.00. Discount fare tickets are released about 12 weeks in advance. (If you book farther ahead than that, you'll pay full fare.) Book at www.eastcoast.co.uk to get a fare as low as £14.50. You can take bus #555 from Bodwin Parkway to Wadebridge and bus #594 from there to Tintagel. See the timetables here. (Disclaimer: I haven't traveled this route. I found the timetables by doing a Google search.)

Julia - I think you'd make out best if you rented a car in Bodmin (or some other place such as Exeter). Travelling around Cornwall to places like Tintagel, Port Isaac, Padstow, Eden Project, etc. is infinitely easier (and cheaper, and more efficient time-wise). We bought our train tickets as indicated above well in advance on the website http://www.firstgreatwestern.co.uk. Very reasonable if you book far in advance.

Thanks! Bodmin Parkway. I was not clear in my posting about after that - some friends will pick us up in their car at the station and take us to Tintagel. Helpful to know the wide range of train prices. Thanks to all. Julia