You've got nothing to lose but it'll depend on the job, location and seniority.

Where the missus is, also a bank, they have a set band that is x number of days for y position and experience. There is no negotiating until you get to the VP kind of level. Salaries are banded too, but there's more wiggle room to negotiate that.

I have found the same thing in America, whatever someone promises something not in writing, it can be broken whether by the person making the promise, or a new manager. And some companies are very leery of granting extra benefits verbally in case other employees use that to show discrimination.

I ran a company in an area hunting was big, we routinely would give men the day off for the first day of hunting season ( we would alternate, half the men one year. half the other year). Company lawyer went ballistic when he realized this, and said we were just a sitting duck for a discrimination lawsuit.

I have found that for some senior or critical positons, easier for manager just to give the extra days than to try to get through payroll systems or get a written exception.

In my line of work it (IT) it seems that holiday time is increasing. I see people often saying they get 10 days but all the jobs I applied for gave me a minimum 15 days and now I have negotiated more.