[dmi]

I had exactly the same situation while at a startup, which got spotted by myself ("Woah why did my taxes jump?") and my employer ("What's this weird tax code?"). Getting it fixed with HMRC was almost trivial, and the refund was very nice.

Years later, I moved to the US. I'm sure it's probably less bad if you've grown up with this system, but I still have no idea what to change (or by how much) to make sure I don't owe taxes next year. I don't understand how to play the tax game, so I'm pretty sure I'm paying much more than I should.

However, a friend has an interestingly different approach: pay as little as possible in taxes throughout the year, set aside what you estimate your tax burden will be, keep the interest, and pay it all in one lump sum at tax time. They refer to this as "not giving the government an interest-free loan".

Edited to add: I never thought I'd say this, but... I miss HMRC.