just try bleach fill a sprayer with diluted bleach spray and give it a good old spray leave it a while. if it is safe use a pressure washer to wash it off with. if not a hose pipe should do it. make sure no plants are in it at the time of doing it.

I use a moss/mould killer rinse, I spray and leave for at least 24 hours before cleaning with a brush on the hosepipe (I found with bleach it stayed there and then turned black. It may be too late to do now if frosts or very cold weather is forecast?

thanks, I will try the moss killer I think.