I knew I'd read about it in a newspaper article some time ago, but had forgotten the details until my dad mentioned that he had booked train tickets to visit friends in Truro from Stafford through www.splityourticket.co.uk/ last month. 
He told me he'd saved over £50 simply by using the website, which breaks down the journey into lots of smaller trips, thereby benefiting from the cheapest possible fares on each portion of the journey. Split ticketing or buying separate tickets for individual legs of your journey instead of buying one standard through ticket can often be cheaper and is completely legal and above board under condition 19 of the National Rail Conditions of Carriage on the condition that the trains you travel on stop at all the places where the tickets are split. The downside? The only one two my Dad could think of were the fact that he had 42 separate tickets to cover the journey (between them, not each)! and the problem that through seat reservations were not possible, so they had to move seats a few times whilst en route. But with good organisational skills, this was a small price to pay!
I decided to put the site to the test myself, and looked up the equivalent off-peak journey from Stafford to Truro return for weekdays in August with a senior railcard (which my parents have). For two people with the railcard, www.thetrainline.com offered off-peak tickets booked in advance at £195.50 whilst www.splityourticket.co.uk found a fare of £128 for exactly the same journey. You can still book the whole journey online, have your tickets sent to you in advance or pick them up at the station, just as with 'normal' booking sites, so it's just as easy to use. So, if you're planning on going any distance on the train this summer (or any other time!) I'd highly recommend you check www.splityourticket.co.uk/ first, as you could save £££s!