Overpaid tax - they won't give it back

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I've been in my current job for 10 years now and when I started, it seems that they didn't record my tax details correctly so I had the wrong tax allowance for years. Payroll discovered this and I was eventually paid back six years of arrears which was nice. However they will not pay me back anything more than 6 years saying that's all they are obliged to do and that I should heve sorted it out earlier. My problem was that they had given me a tax code even though it was wrong and I had no way of knowing. Does anyone have any idea of waht I can do to get the other money they owe me? Thanks