Cheap Train Tickets

From memory, there's a website that will work out the cheapest train fare for you by piecing together multiple journeys.

For example, if I went from Durham to London it'll suggest something like

Durham to York York to Leeds Leeds to London

So I buy 3 sets of tickets for a journey as it's cheaper than straight Durham to London.

What is that website?

If it doesn't exist anymore, I'm 40 and wanting the cheapest way to return from Durham to London once a month.

Any suggestions?

Thank you in advance