The one off bill comes in approx 18 months after payment of the FRI. It is caused by the fact that once you earn over £100000 in a tax year you start to lose your tax free allowance, and it tapers down to a zero tax free allowance for those earning over £120000. Therefore, with an £80000 or £100000 FRI You will be over the £120000 limit and will receive a tax bill the following tax year for 40% of £8000 (your tax free allowance), with a retrospective tax code to reflect that. It caught me by surprise, but thankfully I had not spent the whole amount by then.