Well, the lion dung and ultra-sonic waves did indeed deter the neighbourhood cats – at least for a few months. But the problem with both those solutions is that they don't last – batteries run out and monthly deliveries of lion dung get expensive. I had hoped that the shitty kitty would have learned to do its business elsewhere (ideally its owner's own garden) but in the last couple of weeks the stinky turds have returned with a vengeance.
Time for another solution, proposed by Rich. Extra hot chilli. It irritates their paws, but doesn't harm other wildlife or plants. And happily it's easily available in large quantities from our exotic corner shop.
After some discussion with the man in the corner shop, we plumped for the extra-hot chilli flakes. Powder would quickly blow away or get washed away in the rain, while the whole chillies would not have such good coverage.
We have given the garden a first generous sprinkling, and now have to wait and see...