Well bein a manager here's soomething you could say if your doing in excess of 20 hours a week: its a violation of the working time directive.
48hours weekly for those in a perm position.
20 hours for students. (i believe you are still in education?)
their the laws in the UK anyway,
on the short notice thing....you could have a quiet word with your manager saying that the short notice their giving is making covering the shifts a large liability, as sometimes you won't be available and they need to understand that.
hope that helps.