The best fare is a £17.00 Advance single - subject to availability, of course.
If you're after the best combination of tickets, then the best thing to do is look at splitting en-route but it really depends on your own requirements and how much additional journey time you can spare to save a few quid. Best isn't easy to define. One day cheapest may be possible to calculate by a computer program - but that's some way off (but I am informed that people are working on it!)