Please, please, please don't use the dreaded Trainline.com. They can't offer tickets any more cheaply than the National Rail website and will probably actually cost you more (through booking fees).
Further, I used to run a railway station and I got fed up of having to explain to customers that tickets they'd been sold by Trainline.com weren't valid on any train. (e.g. because they'd been issued using a promotion offered by company A on a train operated by company B, with 'reservations' on a service that didn't even have seat numbers, yet alone reservations!).
Use the National Rail website and nowhere else!
http://www.nationalrail.co.uk/
Alternatively (if you really want to make sure that you've not missed any bargains) check on the websites of the relevant rail companies which, in this case, are Virgin Trains
http://www.virgintrains.co.uk/
and London Midland
http://www.londonmidland.com/