A colleague asked me to look at his tax code. He owes about £500 in tax, so it has been reduced by 250 - being 5 x the amount.
However, as he is a higher rate tax payer I explained that this would mean that in fact pay roughly double.
He phoned HMRC who said (paraphrasing) that the tax code change only affected the 20% band, and the 40% band kicked in at a fixed monetary amount so the tax collected would be fine.
Have I really got myself in a pickle, are they right? (I don't think so.. but am happy to be corrected)
My payroll software works in bands of income.