Contractually you would be entitled to expect to work up to 43 hours per week, although you could also be asked to take on extra hours as required by the business. There are certain legal limits, such as no more than 48 hours per week, but strictly speaking they could ask you to work more than what was agreed.
There is actually no legal right to be paid for overtime unless the contract allowed you to be paid an overtime rate. So many jobs pay a salary for a certain number of hours, yet the employees work more than that for no additional pay.
Also, your rights are somewhat limited due to the length of service you have with the employer. Until you have been there for at least 2 years you are not protected against unfair dismissal or constructive dismissal. This means that you could be forced out of a job due to the unreasonable behaviour of the employer and not be able to claim against them. Similarly, they could dismiss you if you do not accept the requests they make and you can also not challenge the dismissal. There is nothing stopping you from negotiating with them over this but if they flatly refuse to budge, you have to make a choice – either accept the changes and keep your job, or refuse to do the hours and risk dismissal, which as mentioned you cannot challenge.
I'm sorry if this is not necessarily the answer you were hoping for, however I do have a duty to be honest and explain the law as it actually stands. This does mean delivering bad news from time to time. I hope you understand and would be happy to provide any further clarification if needed. If you are still satisfied with the level of service you have received I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 starts at the top of the page. Thank you