Wrong tax code?

BoiledBreadTomatoSauce

It's nearly £8k above my actual salary and my account says that I've already underpaid by £245 in 2019 - 2020 as they're estimating it on a £45k salary.

I think I've worked out the 2018-2019 issue. It's a combination of an overlapping pay period between old and new job, plus the 4-weekly pay cycle in old job meaning I got paid 14 times in 2018-2019 as opposed to the normal 13 (it happens once in a blue moon for jobs paid 4-weekly)

t0rt0ise

The estimate on the web doesn't matter. Mine says they think I'll earn 20 grand more than I will. Where they got the idea from I have no idea. But I keep a spreadsheet with my income detailed so that I can see if the tax taken is right by the end of the year. At the moment I'm paying toomuch tax as I've just taken early retirement but I expect it to work out in a month or so.