If you're stopping in London or travelling thru on the way up to Stevenage and are catching the train from Kings Cross to get to the match then if you travel in groups of four you can save half the fare if you ask for a Groupsave ticket when booking (the ticket office staff won't tell you). The Groupsave ticket allows 4 people to travel for the price of 2 if they stay together as a group. A day return from London to Stevenage is £14.. Four travelling together can go for £28.
National Rail Enquiries - Special Offer - GroupSave