Whilst at work I received a call from school asking me to come and collect my son as he was being violently sick. I ask my employer if I could go and they agreed with my request for emergency dependent leave. I only worked six hours of my shift and returned to work the following day. Later in the week, I got a phone call late evening asking me to come into work on my day off to help as they were short staffed. My rate of overtime is 1.5 x pay. When I got paid the following week my employer deducted the hours I had taken off for my son from my overtime rather than my basic pay. My understanding is that they cannot do this and that I have a legal right to emergency time off to deal with a dependent. I obviously do not expect to be paid for the missing six hours, but surely I am legally entitled to my normal overtime rate for the day extra.
Payment for overtime will be determined by your contract. Some contracts provide for a higher rate (such as 1.5) for hours worked as overtime. Many of these contracts also give a minimum number of hours to be worked before which the higher rate is paid. In other words, overtime rates are not just paid for hours that are not part of your usual shifts, but only after the minimum number of hours have been worked that week. It may be that your employer has applied this rule to you before calculating your pay.
You should check what your contract says about overtime and whether it stipulates a minimum number of hours before the overtime rate is payable. If it does, your employer can legitimately pay you at your usual rate for this minimum number of hours worked (i.e. deducting the unpaid emergency leave) before the overtime rate is payable. If it does not, you should be paid at the usual overtime rate.