Smell of cat poo in garden

Given the glorious weather of late we have been sitting in the garden and have noticed a strong smell of what I think is cat poo. Don't get me wrong, we both love cats and have never minded their visits to our garden. We had the family over yesterday and the smell was really quite pungent. We have looked around and can't find anything obvious where it hasn't been buried. Does anyone have any ideas how to get rid of the smell or what to put down to deter the cats using our garden as a toilet.