I'm going to go against the grain here and say you can lose money.
Incorrectly following the instructions, not understanding what you are doing at the bookies/exchange/etc and not knowing about the unique rules of certain sports (horses/tennis/etc) can lead you to lose cash.
If you have more money to start MB then great, it will help you do more bets at the same time. This will allow you to make more money, faster.
HOWEVER.
Don't rush into MB. Take your time, work with a partner like Profit Accumulator or one of the others and let yourself be hand-holded all the way through the process.
Start with the £50 (or £100) and go from there. Once you've made back double this, if you need to take your £50 (or £100) out and then everything you do will cause you to lose none of your original stake. Lots of people make a considerable sum with MB. There is no reason why you can't be one of them.
Just take your time, be patient, and understand what you are doing every step of the way.
My final bit of advice is that if you have a gambling problem the don't do MB. There are other ways to make similar amounts of money (not for the reduced timeframe that MB has) so investigate those.
The Reverend