The plot thickens. Just realised that my Teacher's Pension, which is for a modest amount and not something I receive a payslip for, has had the emergency tax code k993 applied to it and has now been adjusted to 652L. This is most likely because I left HMPPS in October so am no longer earning as much, in fact considerably less.
This action has resulted in a tax refund that I wasn't anticipating and more or less cancels out the arrears due at the end of the month.
This of course is great news, but by fuck, does it really need to be so stressful? My earnings are all visible in real-time to HMRC and yet it takes them up to 18 months to spot fluctuating wages and to make adjustments. So much for IT and AI in particular set to help with things like this.
For those that don't know, tax arrears are no longer recovered via tax code adjustments, you have to either cough it up in a oner, or call them once it's due and to setup a payment plan.