When you hear from them a 2nd time: Write- State that your own records do not show an error (dispute). State you write in good faith, however, for you to enter into further correspondence they must confirm the specific nature of the error they have made and supply documentary evidence of the alleged debt. This must document the adjustments. Relevant tax treatment or deductions will be considered on a net basis only as this tax has already been paid. To consider the matter further you require these to be submitted within 28 days. Each item of correspondence over or above this confirmation will be subject to your admin charge of 25quid to cover your materials and time concerned.
If it’s <1k you will not hear from them again I bet.
I do know someone this happened to – the big co. really struggled to provide documentary evidence of the adjustments and it was >10k. He did pay (over time) cos he was a good egg, however they could not have taken it further.