[ENGLAND] Not getting contracted hours, what are my rights? (21M)

I currently work in fast food, started working here last year in August. I was given my contracted hours, which is 24, when I first started but my hours were reduced a month later when I started college due to a change in my flexibility. This is what my contract says regarding my ohours of work:

"Your basic working week is 24 hours. However, there may be occasions where the number of hours required is more or less than this according to business need."

Am I supposed to be given 24 hours or is my employer allowed to give me less?