Thanks everybody.
Anyway, the feasibility report.
the water ornament should indeed be fixated somehow, to guard it against an occasional bypasser who might enjoy to own it. The bistro set has been fixed with steel wire as can be seen on the pic. Sure you can cut that, but it requires dedication and will leave traces of the criminal act, for the police and ensurance. Random bypassers, like pub customers, late at night, will likely not carry the equipment for that. But they are welcome to take a seat.
The water ornament however, cannot be fixed so easily. It's polyester and the lower part is the reservoir. Cementing it onto something adds the complication that it can't be removed easily during winter time, as per recommendation of the producer.
But the back yard has high fences around it and it's the idea to make a little pond there in autumn, when I can move the roses, and maybe it's an idea to make the ornament part of it somehow.
Anyway, the next project is a little home for Gizmo as it keeps raining all the time and he wants to be outside, no matter what.