I had the same agreement in place with my ex some years ago, It was our place pretty much when we were together, but my name was on everything, so when we split I pretty much had ownership and she was there out of courtesy...
The rule was a simple one. She can stay aslong as she didnt bring people back to the flat, and I would do the same. I dont want to here her at it with someone else, and vice versa.
Long story short, I finished work early one day and found a bike parked up outside my flat. Now I wasnt in so, fair enough. it shouldnt be that much of a problem, but it became a problem when i noticed my bedroom light was on. You see, She had been sleeping on a temporary bed in the living room.... So they had been in my bedroom, on my bed. So I was ****** off.
I knocked on the door(yes my own front door), and shouted so she knew it was me. She came and answered looking sheepish. I said I was going for a walk, and he had best be gone when I get back. I nipped to the local takeaway, got a can of coke. Drank it on the way back and when I arrived home, Bike was gone.
Thats when I gave her a week to find somewhere else to live, and she did.
Fin.