Thanks all for the advice - although the general consensus seems to be to not let her onto the balcony at all, which is what I was afraid of! Poor cat is so noisy all the time where we live at the moment, I'm sure it's because she wants to be outside. We just couldn't get a villa with a garden so I was kind of hoping that she'd enjoy the fresh air of the balcony instead....maybe I'll see how it goes with just keeping her inside to begin with then, seems that would be a safer option. Anyway the apartment is much bigger so she will have space to run around inside. As an alternative I might try the Kitty Walk though!
