Thank you. When you do a specific role there is no guarantee you will be paid a specific salary, even if others doing that same job receive it. You are only entitled to receive whatever you have agreed with the employer. So if you started working in the new role with no pay figure agreed, then it would be rather difficult to argue that you should have received anything else than what you were already on.
Some employees would try and argue that they should simply be paid what others doing that job are doing. However, the law on equal pay is frequently misunderstood. It is quite common to believe that there is a right to equal pay across the workforce, especially for workers that perform the same or similar jobs. Legislation on equal pay was actually brought in to deal with the gender pay gap and to ensure that women were not paid less than their male counterparts because of their gender. So that is why equal pay would only be a relevant argument if the reasons for the difference in pay were gender-related.
The reality is that employers are free to pay their employees whatever they deem appropriate, as long as it is above the current National Minimum Wage and in accordance with the employee's contract of employment. It is not unlawful to pay employees doing the same or similar jobs different rates. As mentioned, this would only be an issue if there is a gender-related reason behind this.
So unless there were clear gender-related reasons for the difference in pay, which cannot be justified by any of the above reasons, there is nothing illegal in paying different rates even if the workers are performing the same job. I agree that is appears unfair and morally wrong, but unfortunately it is not illegal.
Therefore, in your case you would only realistically be able to claim the higher pay from the point where it was agreed that you would get it and you won’t be able to back date it without the employer’s consent to do so.
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 actual legal position. 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 I have provided regardless of the contents of the answer, I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 stars above. Thank you