How did you get the planters and the cat up there?

I did like the other suggestion. Get a BB or pellet gun and shoot the dang thing! You'll have hours of fun and when the plasterer comes to fix the holes in the ceiling, he can change the battery...

Do you know any "really tall people" ?

Real answer is find a neighbor who has a similar home design. (let me guess that it's not a one of a kind?) and ask if you can borrow the step ladder that they use to change the battery in their smoke detector?

You are going to need a ladder, even one section of an extension ladder would work. Change the battery once a year before it goes wacky on you.

Cheers!