If you spend on cards you can often download into a spreadsheet including categories of spends (fuel, supermarket etc). Then analyse what you spend where. Like others have said, it's easy to spend hundreds on food out of the house or takeaways.
Very extravagant if you don't have the spare money and not on to be cadging off your mum because you can't be arsed to make a packed lunch if this is what a lot of the money is going on.
If you haven't done it before, it's always worth systematically looking to reduce your bills by changing gas and electric suppliers and reducing your mobile phone and TV contracts etc. All these companies try to get you to spend a bit extra with them and if you're paying full price it's too much. Loyalty or introductory discounts are always available.
Always put money aside for annual and unpredictable expenses. Lots of people say their finances were derailed by an unexpected bill, but if you have a car, pet, washing machine or house, these will cost money sooner or later.
Do you have lots of food in your cupboards and freezer? One way to stretch that £50 is to aim to eat everything up and only buy the minimum bread, milk, fruit and veg (cheap seasonal stuff not fancy expensive stuff) you can even if it means eating slightly weird combinations. I've had pork pie, gravy and broccoli for lunch twice this week because I'm trying to clear out the crap from the freezer graveyard and it was quite nice actually.