Getting TRS when I shouldn't.

I just discovered that I've been getting TRS on a non principle private residence when I shouldn't have. I didn't realise that when I moved out I should have stopped the TRS on this property.
I'm wondering whats the best way to go about this?

1. Ring Revenue and inform them of my mistake and pay back the monies owed. (It could amount to a fair bit and I really don't have the money but I'm sure some kind of payment arrangement could be agreed with them. I also imagine that the monies owed will have interest attached to it?)
or
2. Fill in the TRS de-registration form and say it ceased to be NPPR this month. (I reckon i'm playing with fire with this option)

Anyone else in the same predicament?
Cheers.

Go back to Revenue and tell them that you made an honest mistake and weren't aware that you had to stop the TRS. You will have to pay it back, I imagine, but they should come up with a plan acceptable to both of you to pay it back. Did you buy a new house? If so you should have moved your TRS to that one. You may be owed TRS for that house. I'm not fully sure of how that would work, someone more knowledgeable may be able to help you on that.