former employer incorrectly calculated holiday entitlement

Hi,
I got made redundant in August and received my severance pay yesterday. It was about £2000 less than I was expecting. When I asked someone from HR why this was they said I took 6 days more holiday than I was entitled to and they had deducted that from my final pay.
I actually took 6.5 days less holiday than I was entitled to and have a copy of the holiday spreadsheet that my manager filled out. He has claimed to HR that I took 20 days holiday when I only took 10, and the holiday spreadsheet states that I took 10 days.
I have sent my holiday dates to HR telling them their calculations are wrong but they have not replied to me.
I don't know what to do now, I was relying on getting that money. We confirmed verbally in a meeting how many days holiday I was being paid for and they didn't contact me to tell me that this had changed.
Sorry about the long post but I don't know what to do. Any advice?
Thanks for your help.

Thank you for your question.

If you have written to your former employer and they haven't responded then the next thing is to make a claim to an employment tribunal under section 23(1), ERA 1996 for Unlawful Deduction from wages. If the tribunal upholds your claim, your employer will be ordered to repay to the you the amount unlawfully deducted.

For more information and to download the ET1 application form