Never use The Train Line. They charge a booking fee. Just your local train operator, they all use exactly the same system, same database, etc but you won't pay £1 to buy the ticket.
It's worth it for me. Email me for details. I will be slow to reply, but I will reply.
There's an app for that: https://itunes.apple.com/gb/app/tickety-split/id513845919?mt...
Mobile web version: http://splitticket.moneysavingexpert.com/tool.php
Guide: http://www.moneysavingexpert.com/travel/cheap-train-tickets
Probably because (as I've discovered) it's hard.
But when you have to deal with things like (eg, I'm hope I'm remembering this right from someone I know) "EUS-CDF being more expensive than EUS-SOT + SOT-CDF even though both halves are the same train as the EUS-CDF journey", you can see how no-one has a bloody clue.
I must confess I've never used the app. I just read about it in the MSE email newsletter.
I grew up in London, and didn't often have reason to take a long distance train. I have a vague childhood memory of discovering that London and England weren't synonymous.