Paid wrong for 2.5 years now they wont pay up properly?

[bradfdan87]

my partner has recently discovered that over the past two and a half years she has been underpaid for every hour of overtime she has worked. Rather than paying her 1.5X her hourly rate they have been paying her at 1.5X a smaller rate by mistake. it was a mistake made on there end when she moved from temp to permanent. we have calculated the error to over £1700 before tax. it has been two weeks since this has been discovered and the company have admitted responsibility and are willing to pay. However they would like to pay the amount for this tax year to her immediately and the rest in small instalments over a period of time this tax year calculates to about£500. The reason given is it will affect there budget. bare in mind the company turnover is in excess of £250million. We understand there are complex issues to be resolved including back tax and are willing to wait a reasonable amount of time for these to be resolved but would like to bed paid owed amount ASAP rather than have to wait to help there budget. where does she stand what are her rights and what can we do if they refuse to change there mind?
Additional Details
her payslip's were not clear enough. they just showed hours worked and total pay received. we did check every payslip that she got paid the correct amount of hours for the correct rates. as the difference was only about £1 per hour it wasn't noticeable until she compared her wage slip with a colleges.

[factor-fiction]

I've just answered this in Business and Finance. It is confusing and leads to fragmented answers when posts are duplicated. Might be better to close one down by posting a message saying don't reply, use this link.

[albalass]

phone ACAS, they deal with employment legislation 0845 747 4747. they'll be able to explain her rights and where she stands legally.

[bradfdan87]

Great help Many thanks Albalass.

[buildersmate]

However we effectively provide the same service here - plus the titbits ACAS won't suggest.