Unpaid overtime
A. There is no statutory right to be paid for overtime that you have worked; this is something that is governed by the terms in your employment contract. You will only be entitled to be paid for additional hours if you can establish that you have a contractual obligation to work overtime.
Since it appears that there was no agreement for you to work the additional hours, and it was to cover unusual circumstances rather than being part of a fixed pattern, it’s likely that the overtime would be regarded legally speaking as “voluntary”, no matter how much the work needed to be done.
As such, your employer has no obligation to pay you for the extra work.
However, your manager’s stance hardly fosters good employment relations. It may be that your union rep will be able to help you negotiate an “ex gratia” payment. This would recognise the additional work you carried out without setting a precedent.