You should normally expect an employer to offer more pay when they are asking you to perform work over and above the normal duties, particularly if this involves extra hours.
However, your employer may refer you to an apparent exemption for this within your written contract of employment – e.g. “the employee agrees to temporarily undertake such other reasonable duties as the employer may from time to time request”. Bear in mind that even if there is such a vague clause, it is not necessarily conclusive. If your employer is asking you to take on substantial extra duties indefinitely this may amount to a variation of your contract, which would entitle you to renegotiate the terms, including pay. Seek advice from your union rep.
