My understanding is you pay the rate applicable to what your earnings are.
So if your a lowly paid P/Person earning 26000 p/a you would pay normal rate. If towards the end of the year with overtime you moved into the next tax bracket ( which i believe is at 33000 and above- to 40%) you pay that rate on whats over that banding.
My gross for the year appears every month on my wage slip, going up as the year goes on.
All i know is that as you get to the end of the financial year say jan/ feb/mar monthly wages if Overtime is offered and due to your earning so far in the year you are aproaching the level of the next tax bracket, you will pay the higher tax on those earnings putting your gross above that level.
This may well be as a result of what your annual salary is, you cannot predict the extra like overtime.