I'm going to assume that you're a first year student, but if I'm wrong I apologise for that.
First you need to deduct your rent from your matinence loan. This might leave you with some money left over, or it might go into your savings like mine did. From here you take whatever money is or is not left, see how many weeks until your next payment comes in, and divide the amount left by the number of weeks. This is the easiest way to find out how much you have to spend.
I personally only normally spend £20 a week, if that, normally with laundry costs included. Granted I don't really drink or go out, but consider where you spend your money when it comes to food. Try and avoid takeouts and ready meals, and learn how to cook. You could get a single serving lasagna for £2, or get some meat, pasta sheets, red and white sauce for about £4-£6 and end up with 6 servings. Thats a £8-£6 saving if you kept buying that £2 lasagna. Same goes for shepards pie and what not; it all freezes perfectly well.
Ultimately just make sure you're not overspending. Try to go one or two days a week without spending anything if you can, see if you and your mates can batch cook to spread out the cost of food, and never buy lunch on campus. My dad is the line manager for a company which makes a bunch of those two go sandwiches: costs them pennies to make, and they charge you pounds.
So ultimately £50 a week sounds like a lot to me when, if you're smart, you could probably get away with £30 a week and put that extra £20 away for a rainy day or a night out.