I have worked for my current employer for nearly 10 years.
My employer, since I joined the company 10 years ago, has always operated four separate limited companies.
I joined the Company A Limited, any letters to me from my employer are addressed from Company A Limited, and I've always been paid by Company A Limited.
My employer also operates four separate limited companies, Company B, Company C, and Company D Limited. At the start it wasn't made clear that I would also be supporting these separate companies but over time I've learned to manage and, even though it sometimes gets to me that I am practically working for free for those companies, as my employer isn't paying an IT person to support them, I've generally always let it pass.
My employer is now opening an additional 3 separate limited companies and I have been involved in the process from the start. At no point has my employer mentioned that I would be getting a substantial pay review, or, if I wouldn't be supporting them long term, a one of bonus for the extensive project management work that I'm doing. I've raised this with them and they've said that it'll be reviewed once the new companies are operational, but that doesn't stand with me as I'm clearly already severely underpaid for the 4 companies that I technically already work for, without the additional 3.
I did mention this off the cuff to our HR person and he basically said that once you've done certain things for a certain time period, even though it's not the job that you joined to do and it's not in your contract, it would legally stand for them and basically becomes your contract (is that even correct?).
What should I do here? Am I able to put my foot down and say "next Monday I am coming in to work, to work for the company that pays me, Company A Limited, and I will only be working for that company, not the other 3 that I've covered for nearly 10 years, and definitely not your 3 new companies"?
What's the most professional way to handle this? Should I deal with the fact that I'm technically not paid for the 3 existing additional companies that I cover, or just deal with the new 3 that I will be expected to cover in the future?