Employment law advise regarding overpayment of maternity pay

Industry: Internet/Software
Legal topic: Employment
Description of legal work: Hi, looking for someone with legal qualifications to provide advice for the below please.

My wife came back from maternity leave 3 years ago. During maternity she received the statutory maternity pay, but her employer paid her an additional amount (around 250 per month) as part of the company standard maternity offering. This payment was to compensate for the fact she had no salary and would otherwise be unable to process her childcare vouchers. When she returned to work her HR department forgot to cancel a portion of her maternity pay.

As a result she has been overpaid by about 250 a month since 2013. At no stage during maternity was she told she would receive this extra amount, the only time she found out about it was when HR called her to advise she had been overpaid.

Some other things to consider:
- HR stopped paying her the overpayment 3 months before any contact was made regarding recovery of the overpayment. I believe they have a legal right to notify of any change in salary?
- They are requesting the full amount back from her - over £5k
- She is still in employment with this company.

I have read online that employees are not covered by the Employment Rights act of 1996, and in fact the employer has the right to take back the money.

However we want to challenge this either under the rules of estoppel or the change of position defense. My arguments are:
Estoppel:
- Employer must make a representation suggesting the employee was entitled to the money... She received payslips each month with this additional payment, suggesting she was entitled to it. Not once within the 3 year period did the employer come back to her advising of this issue. It is not included on the maternity documentation nor did they discuss the overpayment at any point.
- Employee has changed position as a result in good faith... My wife has spent the money - mainly on childcare etc. We can provide bank slips confirming this.
- The overpayment must not have been down to the employees fault in the first place... This is self explanatory, especially considering the first point about my wife not even being told about the payment.

I believe there is also a rule that employers must not take payments if it will cause financial hardship. Again we can show bank statements to show my wife is always at or in her overdraft... we pay over £1000 a month in childcare so we have no spare money for HR to start taking it back!

Thanks in advance.
Extra notes: