Just leave it. You've got the extra money and, because of your greater responsibilities, you'll be in a better position to apply for promotion (or to seek another job elsewhere) when the opportunity arises.
As long as they avoid 'statutory discrimination' (e.g. by paying mem more than women) employers are free to have entirely different contracts with each of their employees unless, say, they've entered into an agreement otherwise with a trade union. (i.e. there's no legal right to 'fairness' in general).
For example, in the City of London it's not unknown for two bank employees, doing exactly the same jobs and sitting adjacent to each other in the same office, to have vastly different salaries. A guy who's been there a long time, without pushing for promotion, might be on £40k p.a., while his colleague (who was head-hunted by the firm) could well be on over £200k. It's perfectly legal.