We asked exactly the same question at our garden centre the other week. Armillatox was mentioned, but then the assistant asked if there was any vegetation nearby, ie did the path border on to a flower bed. If NOT, then it was suggested that the cheapest way would be to buy the cheapest bleach available, get a wire brush to brush it in with and then use a hose pipe to spray it it off. Now, that's not an option for us and also it sounds like hard work, depending on how much you have to do. So, it's over to you!