sleeping bags in hostels

heya,
I'm going to be staying in a hostel in lusaka during my stay in zambia and i was told that i would need a sleeping bag for winter/spring because it gets very cold at night. I was just wondering if this was true and also any sleeping bags which you can recommend if i do need one. I am on a budget and don't really want to spend more than £40.
Thanks
sonal
xxxxx