I'm fully aware that the amount (IF it is accurate - and I have reason to believe it isn't) will need to be paid back. However (a) they could simply have contacted me, not immediately referred to a 3rd party, particularly a debt collection agency which could impact my credit score and (b) they should have provided the information for the calculation (i.e. my payslip) long ago. They need not have been so heavy handed, and this reflects badly on them as a company. Ex-employees (and their friends and families) are potential ex-customers too as a result of this, nor would I recommend anyone too them as a potential employer. Whilst one person's contacts hardly makes a difference to their bottom line (min. of 20 x 20 people each they will tell), multiply that up by all the other ex-employees with incorrect pay calculations over the last few years and for a company with such a high turnover of people, that's a lot of the UK population influenced. What goes around, comes around.

I'm in a similar situation. 3 months later, ex-employer claiming I've been overpaid & they still haven't sent me my final payslip despite written requests. Temporarily looking after mother full-time whilst I find a job closer to her. Like you I have some money (from sale of house, intended for deposit on house locally & to keep me going until I do find work - as I resigned I think I'm not entitled to state benefits?). I've written to them today (and the debt collection agency they immediately washed their hands to) explaining I need more evidence of the overpayment (especially as they failed to make my final payment on time in the first place due to another "payroll error". Good luck with this - everything seems to be stacked on the employer's side nowadays.