Unless contractual provisions apply, every employee's contract is entirely separate to that of any other employee. (There's an exception relating to the rights of part-time staff but that doesn't seem to apply in your case).
In the City of London, there are people working alongside each other in the same office, doing the same job, where one employee might be on ten (or more) times the pay of his colleague, simply because one person joined at the bottom of the pay scale and has never pushed for more money, while his colleague has been 'head-hunted' from another company. It's perfectly legal, and long may it remain so.
Chris