Salary Overpayment - Employer/Bank Protocol

advice,_please_4

I worked for a borough council until the end of March. I received my March payslip upon leaving (via workplace) and P60 but have yet to receive a P45. My salary was paid in to a main bank. During my employment, a regular monthly salary deduction was made in favour of Credit Union (same borough).

Last week, whilst making a withdrawal from Credit Union, they advised me that “payroll overpaid your salary in April and have just re-called the monthly equivalent of salary deduction” and that Credit Union have returned the money to payroll. I have since received notification from borough that “we overpaid your April salary”. I am now checking the ‘overpaid salary’ with my main bank but need some advice:

The first I learnt of the salary overpayment was through Credit Union. The payroll department have now written to me acknowledging that they had failed to notify me of the overpayment due to “clerical error”.

Can payroll department re-call money 3 months later from Credit Union / any bank without at least first letting ex-employee know the circumstances and their intentions?

Should Credit Union be giving money back three months after overpayment date and without checking with CU member first?

Is this because of their inter-borough association? If I worked in the local book shop and similar circumstances occurred, could the book shop recall the money from a bank three months later without first advising the ex-employee?
Aren’t there FSA rules?

Have arrived home today and opened “legal notice” letter from council recovery department.

If it transpires that I owe the money then of course I should pay but I am extremely concerned about the Credit Union / payroll department interaction.

Also, depending on my main account balance I may not be able to re-pay the amount in full – I had planned an employment break, have not had much dealings with my main bank, instead using my Credit Union account and did not realise that there may have been extra money in my main account – the ‘overpayment’ amount would not have been quickly identifiable. Is a monthly repayment offer acceptable? If so, over what period? Thank You