Poor you OP!
If you've tried to resolve this for several months, I think nows the time to raise a grievance in accordance with your employer's grievance policy. If you're in a union, ask for their assistance. I'd put something like the following in the grievance.
Dear X
Grievance regarding salary payment
As you know, on date I was overpaid by £x, as a result of a mistake by the payroll team.
The overpayment resulted in:
- a net overpayment to me of £x;
- an additional pension payment of £x;
- increased tax payment and national insurance payments of £x and £x respectively;
- triggered a student loan repayment of £x.
The payroll department have not apologised to me for this error and instead, have been pressuring me to agree with my manager a repayment plan for the gross overpayment amount of £x. I do not agree to repaying the gross amount, as it requires me to repay amounts that I have never actually received into my bank account (I.e the amounts of the tax, national insurance, pension contribution and student loan).
I have had numerous emails with the payroll team about this and have tried without success on several occasions for them to talk to me, but they refuse to talk to me.
I'm finding the situation incredibly stressful, I can't see any likelihood of it being resolved soon, and I am I raising this formal grievance in the hope that you can please help for this to be resolved.
I am happy to return the amount of £x, which is the net amount that I received into my bank account. I am happy if next month, my pension contribution is reduced by £x, to reflect the amount of the overpayment into my pension. I cannot agree to return the tax, national insurance or student loan payments as I never received those amounts into my bank account and if I were to pay them to employer's name, it would put me in a worse financial position, for a mistake that I did not make, and which I have been trying yo resolve, without success.
I look forward to hearing from you and very much hope that you can assist me in resolving this, as the stress and worry is taking a huge toll on me.