Final Pay Deduction, is this right?

[R1 Indy]

Basically i received my final pay last week, after resigning.

There were quite a few errors on said payslip, but my biggest concern was a Credit of over £700 under "Arrears Pensionable" This amount was suspiciously close to the holiday pay I was owed!

first thing i did as i didn't have a clue what this was contact my old boss, she didn't have a clue, so suggested i call payroll.

Now said company is Tesco, so Payroll is in india, so was very hard to get an answer to what this credit was. But turns out:

Around 8 months ago i was paid around £1K as a "sweetener/keeping us quiet" as they were ditching a monthly bonus scheme, i was not happy with this, but accepted £1K was better than nothing!
It now appears this payment was supposed to be the equivalent to 18 months of bonus (it was no where near), and i need to pay back this proportion as i left before 18 months.
I was not told this at the time, and certainly did not sign anything.

So wondering if anyone has any advice on the legality of this??

Is it worth stirring up a bit, or just put down to experience?

cheers