There's some general advice about finding the cheapest train fares in my post, here:
http://www.theanswerbank.co.uk/Travel/Question 277265.html
However, if you want a specific answer please tell us where you're travelling from, with the dates of travel and your flight departure and arrival times. I used to have to find the best fares for rail passengers as part of my job, so if there is any way of reducing your fares, I hope that I can find it for you. (If I don't, I know that the excellent Dzug will).
Chris