I'm sure I don't know it all, so please feel free to add below if you have any other points/tips.
Firstly the site is just like any other train booking site such as http://www.trainline.com
You need to register at http://www.redspottedhanky.com/ and then you can search for and book travel as you would anywhere else. The prices seem competitive, but I still check with virgin trains site now and again to make sure.
The first advantage is that for every pound you spend you get loyalty points and you can quite quickly build these up to cash in for money off future travel. You can also cash them in for other offers similar to clubcard vouchers.
The next big advantage is all the one codes that seem to crop up for the site. MSE, The Telegraph, The Sun and HotUKDeals have all posted codes recently giving away £5 or £10 away at a time. They do all end up on FMUK at some point, but its good to keep a look out.
I tend to add codes to my account and another, say my OH's and book half a journey on one and the return on the other. The free travel soon adds up.
Finally redspottedhanky have recently run a couple of fantasy sports games where it's very easy to earn loyalty points without much knowledge of the sport. Last year there was an Olympic game with a maximum payout of £15 which it was almost impossible not to earn and this year it is Formula 1.
Here the link to the game or it's on the RSH homepage
http://fantasygp.games-redspottedhanky. ... ign=gpgame
Now I know very little about F1, but by picking 4 top drivers and then ignoring it for a few weeks I already have enough points for the max £15 credit at the end of the game.
Plus if you want to take it seriously I believe there are weekly prizes and prizes for the overall winner, guess you never know...
Maybe we could post future codes here as they tend to go pretty quickly.
And, I'm no expert, but feel free to ask me anything if the above isn't totally clear.
Hopefully next year we can all go to FMUKCON for free!
