Income tax doubled mid-year

ender4

chrisbur said:

The need to offer this service is not required by every employee just a few now and then.  During my time in payroll I was always able to explain someone's wages if asked (though had to ask them sometimes to come back later) and that was a weekly payroll with far tighter deadlines than a monthly payroll.
Also the OP advised that "I've asked HR and our payroll people at work, they are stumped and say they they haven't changed anything." not that they did not have time but that they did not know.

It's just because our payroll/finance people just collate the information. They don't actually do the payroll, an external company does that for us. 

BikingBud

Have you tried putting your information and your intention into an online calculator to see how it breaks it down eg:

https://listentotaxman.com/index.php?c=1&yr=2008&exNI=1&age=0&add=0&code=&pension=&time=1&ingr=&vw%5B%5D=yr&vw%5B%5D=mth&vw%5B%5D=wk

or 

https://www.thesalarycalculator.co.uk/twosalaries.php

If you can review these and get 2 answers that are to all intents and purposes the same you could then review where your HR estimate you will be.