More detail here please?

When did you get them and what were they for?

It sounds like there's been a mix-up over what hours you've been paid, but in principal they can withold payment or ask for it back if you've been overpaid. The ball is in your court - although it could be a simple miscommuinication, but it's likely to get messy having been discovered this late.

Edit: didn't we have someone else on here who'd had a dispute with Co-op over their wages? How did that one work out?

I was off sick... i was handing sicknotes in to cover me whilst being off work. I have found out that my former boss had said i was working when i wasn't and never forwarded my sicknotes on to headoffice and payroll.
The cheques turned out to be for hours i had supposedly worked!
i got them for the first 3 months of being off work and i was told i would get SSP for 24 weeks. The Co-Op have since told me i wasn't entitled to SSP... I spoke to a lawyer today and they said its their fault and that i just gotta refuse to pay the money back.
and yes... the last issue with the Co-Op was me.. and it was over the same thing.. boss put me down for hours i never worked and the social wouldn't give me income support. Apparently that was resolved but obviously not as its resurfaced again now but more serious!!!