Perfectly acceptable to pay different rates to staff doing the same job, the company can even have a non disclosure of wages in their contracts, only time it becomes illegal is if it is being sexist, ie employing a man and a woman and paying one of them less because of their sex.
Our company have taken on new guys who are on 2/3rds of what the original staff are on, they keep complaining about it, but they accepted the job at that wage, if they don’t like it they need to either try and change it or find another job.