There was something on the news over here recently Sue about minefield buying train tickets! This might be of some use..
http://www.thisismoney.co.uk/money/holidays/article-1637879/How-cheap-train-fares-simple-buying-rail-tickets.html

and try this too
http://www.moneysavingexpert.com/travel/cheap-train-tickets

Thanks BB! See that both sites say to book 12 weeks in advance, and I did try, last month, but wasn't allowed!!! I'll keep trying, using your links

I have been recommended to use the National Express website. For trains this is now called C2C
http://www.c2c-online.co.uk/
the cheapest is £30 single.....but you have a 3 hour wait in Birmingham
however you can also pay £98 for the same trains...although with the £30 advance fare you MUST get the trains you specify, and with the £98 one you can go on any train.
You can only book up to 13 weeks before travel.
I'm not sure when the advance offers end/start.
(I just bunged in a random date in September)

Hope this helps.