Sweet Arran, mix it 3 parts snow foam 7 parts water.
Low pressure on the foam lance.
And be quick, bottle can empty fast ;)
Leave it for 5 - 10 mins, power wash off from top to bottom.
Few snaps from Truxton yesterday, got of a shot of 200 spittin flames:
https://www.facebook.com/media/set/fbx/?set=a.10150170101569367.317541.503399366&l=9700430344