Cut down on stuff you don't need, drink before you go out, walk the extra 50 meters to Sainsburys rather than Co-op, buy second hand bike parts, transfer all your money to a high interest account... Usual things like that.
Theres no making your £50 turn into £100 after a week, and no way you can make quick easy money otherwise everyone would be doing it.
Get a job doing somethign menial, that'll help.