The strobe and utrasound devices are plain bloody useless Geoff. I had to put netting over my pond to keep cats out and also to stop the heron getting them.

I also fitted a 12 inch high band of plastic mesh around the perimeter with a net over the top on my daughters pond. Bothe have been safe from both cats and herons since being fitted.

Used the old sound and light devices. Not worth a light. Cats just ignore them. I think the problem is that nobody has officially told the cats they are supposed to be deterred by the things.