Typically, you need some money to put on your initial bets and a little bit of a bigger pot to lay the first bets. I would recommend around £50 minimum. You can start with more, and if you do, you can do more offers more quickly without having to wait for the first offer to complete. After your first few completed offers, you will have doubled your funds or better and can start to do more!
Check out my review of Outplayed or to give matched betting a try head over for the free trial.
