train bookings only go as far as june 27th at the moment. but on the assumption that the fares won't change in the intervening 4 days, and that you're only going for the day, you can get a return for just under �13.00, provided you don't leave until 09h25. journey about 1h45-1h55. any train company should be able to do you a similar deal, the one I use is http://www.nationalexpresseastcoast.com/
(then fill in the panel on the left).
The deal that garlandk9 has shown is in fact a walk up ticket available on the day - no need to book.
There are no cheap advance fares on this route so no point in booking ahead. If you travel at peak hours the fare is slightly more - maximum �19 return