took out cyclescheme, made 3 payments, had a promotion after the 2nd one, empoloyer took several months to finally get my pay correct. they appear to have forgotton to tranfer the scheme payments over on last months payslip. where does the onus lie in correcting this.

you owe the money.
would you rather they sorted it out now, or in a big lump sum when they figure it out?

You are only cheating yourself

😉

I would tell them. It is the "right" thing to do.

You have a responsibility to point out the error to your employer. It shows you can be trusted!