Oh I forgot.

Kimlovescats posted this interesting link to a cat proof fencing. It is a net & wood frame which is attached to the top of existing fences. This keeps your cats in and stray cats out.
www.catfencein.com

and you could email Kim for details, as her fella made the same thing himself (clever man)

So if your cats are nervous limiting them to the backgarden might be an idea.

Ps. Make sure your cats are microchipped before you let them out.