% pay rise to manage someone

[Steakpeppersauceandchips]

I’m a Senior Document Controller and have unofficially been managing someone for 6+ months (due to become official from May) Have been a mentor/ trainer to numerous people over the years and will continue to be and most experienced in team (7 others) I work in. I am based in Northern Ireland so appreciate won’t get as high as other locations. My salary in £30k region currently. What % pay rise should I be aiming for? Also have been with company almost 7 years and am only now confident enough to back myself. Am almost 42, not sure that makes a difference. It has been indicated that due to upcoming works the number of people will likely grow.

In some places I’ve worked either you’re on a point on a salary grade and the job either did or didn’t include line management (so you could go from it being in your JD but not having someone to manage to managing someone, without it being a different pay level).

Or a 10% increase to reflect the increased responsibility.

I went from managing no one to 18 staff for £50 extra a month.,,,,3 years in I am £250 a month above someone not managing any staff.
This is public sector so I have zero control over pay structure.
However if I had been in a position to negotiate I’d be asking for at least 10% pay increases fir that. I received ..0.25%. Not enough for the increase in responsibility.