It has to be the Crossbow in Fore Street. (near the old post office)
Nothing fancy just good honest well cooked food.
If I'm in Cornwall, even if I'm not staying in Tintagel, I'll always try and fit in a visit to this restaurant. The chef knows what he/she is doing and their philosophy is never to leave you wanting more - the portions are very generous. If you leave an empty plate they treat it as a sign that they didn't give you enough!