You should normally expect an employer to offer more pay when they are asking you to perform work over and above your normal duties, particularly if this involves extra hours.
What you are entitled to be paid depends on the express terms of your contract of employment.
Also remember that every employment contract contains implied duties of trust and confidence, and good faith. Your employer could breach the contract of employment by requiring you to take on significant extra duties without more pay.
In any event, your average pay must not fall below the National Minimum Wage.
Seek advice from your union rep.