I've been paid a sum of money before Christmas which came with a payslip statng it was a long service award payment.
I was in receipt of this benefit according to my old contract, but stopped working there in September.

I saw no reason to dispute this payment, in fact, I was quite chuffed to revcevie it, thinking, wow, seeing as I'd completed nearly a whole year, they paid me the LSA

Anyway, recorded delivery letter a few weeks later, , they want it back, paid in error etc. and they want it back now.

What are my obligations re paying it back? Do I have to pay in a lump sum or can I offer them installements?