Request pay review for added responsibilities?

[spottygymbag]

I am employed as a part time PA to the MD and Regional HR manager.
Earlier this year the HR manager took voluntary redundancy and his position is being filled internally from January '21. In the interim 4months I have provided cover, along with my counterpart in another country.
Once the new manager starts I will retain some of the new responsibilities, including payroll.
Our salaries are reviewed annually in January-March. Would it be acceptable to discuss a potential increase with my MD in relation to this?
The process is very much a blanket approach to increases with exceptions put forward for review by line managers. So that would be the MD for me given the née HR manager hasn't started yet.
I would appreciate some outside perspective on this and advice to how to approach if it's appropriate.

Some things to think about....
Will you have to work extra hours to complete your PA and Payroll duties. If so you need an extension of contract.
The average rate for payroll administrator £24,191 is lower than a PA rate £27,982. Maybe they will suggest a paycut!
If you are not very busy as a part time PA, is there a risk that instead of reducing your hours, maybe this was a way to keep you busy and justifying your job.
Does your job description include admin duties?
Do you only work exactly to your job description - only doing exactly what has been agreed for the role, with no flexibility?