Payroll messed up my salary

[Shehasadiamondinthesky]

Please can someone tell me where I stand on this.
I got a grade 6 last year in a different trust - February 2000. I asked if I could have the top of the band as I was top band 6 in my previous trust.
They said that's fine and I have the appropriate emails to prove I did everything right and sent the information off, got my line manager to fill in the form and send it to payroll.
We don't get payslips sent to us you have to print them off online if you want them.
I noticed yesterday that they have been paying me bottom of band 6 for the last 18 months and never actioned the form!!!
Now saying they can't possibly refund me as we've been through three tax years and I should have noticed their mistake.
I am normally on top of everything but I had a mental breakdown 18 months ago and was diagnosed with complex trauma and was having hallucinations and hearing voices. I didn't miss any days off work but wasn't on top of my personal affairs as much as normal.
If I have management authorisation in my work emails and the correct forms were sent off to payroll but they didn't action them then it's not my fault, I did everything I was asked to do as did my manager.
That is my money and I am entitled to it.
Can they say no to me legally?

2020 not 2000

Are you in a trades union? They should be able to advise you. Or ACAS.

If it is clear that your employer has agreed to pay you at the top of band 6, they must do so. Mention "unlawful deduction of wages" to payroll. The fact this spans 3 tax years is their problem, not yours.

As long as you submit a claim within 3 months of the last defection then this would form a series of unlawful deductions from wages and is all recoverable. Unlawful deductions claims can only go back 2 years though so don’t let it continue.

Absolutely they should pay you the backdated difference. Cut off is 6 years.

Do you know that payroll definitely received the correct forms?

if you had been overpaid for the past 18 months your employer would expect it to be repaid, please pursue your underpayment, its the responsibility of your payroll department to calculate what you are owed, its what they are paid for - good luck