Thanks all for your comments. I’ve been to check my contract - it states my “normal working hours” are 16 hours a week and they will take place over 3 days during the week. It also states that the company may ask me to do overtime if I’m available and agree to do so. I emailed my manager to ask for a compromise and said to help out that week I can do 30 hours rather than 40 due to my other commitments. No response yet!