I got Overpaid and now they want it back?

Basically, I left my work last month and got my final wage through. Now over a month later they send me a letter saying that they have overpaid me and that I have to pay it all back or they will take me to court. I had no idea that i have been overpaid because it wasnt much anyway and i spent all the money. I left work because the hours were clashing with university and because im pregnant so now i am unemployed and there is no way that I can get that money or even consider paying it in monthly installments because I don't have an income. Are they legally allowed to do that?