I forgot to say that I mostly use Sodium Hypochlorite which is just common household bleach, but I but it in 25 litre drums at a 10% mix. A drum is about €20 from my local farmers co-op, but any pool cleaning place will also supply a similar product. I mix approx 2 parts water to 1 part SH which still gives quite a strong mix,.. simply spray it on and wait a few minutes as the green disappears before your eyes!
Obviously its bleach, so the correct PPE is essential, goggles especially,.. and it'll destroy your nice uniform polo shirt, so waterproofs are required too.
It will damage plants and grass (browns the leaves) if you leave it dry on them, so I always go round the house and wet the surrounding lawns with plain water,.. and after spraying the walls/gutters etc go around again and rinse the lawns/trees/bushes down with water again just in case of splashes.