The cheapest individual fare is the Off Peak Day Return (CDR).
If you want to know the cheapest combination of fares, that is difficult to calculate and almost impossible to guarantee. However there is a "cheapest fare" option at trainscanbecheaper.info, if you've not seen that site yet then I strongly recommend it, though the cheapest may well be rather slow, and you cannot guarantee that the cheapest shown will actually be the cheapest.