I agree with mark dew. Use ordinary tap water to get most of the muck off. Give it a pre-soaking and by the time you've finished soaking the last bit, the first bit will be ready to clean.
You can finish off with pure if you like to get a really excellent finish.
Here are four bits of advice that you don't see very often. They are things that I think are useful and important to know. Others might not agree.
* Take great care cleaning the ridge cappings (things on the ridge) and finials (pointy bits at the end of the ridge).
My technique is to rest the pole on the edge of the roof and move it from side to side and up and down and twist very gently. That way I'm not pushing on them - just letting the bristles brush gently on them.
* To get twigs and leaves out from under the ridge cover. Take the brush off your pole and wrap a microfibre round the end. Then you can just pull the stuff out without damaging the surfaces.
* If the roof spars are very green, you might want to turn your brush so it is parallel with them and scrub along them.
If they have moss growing between the spars and the roof panels, you'll need to use a scraper on a pole to get it off. You can scrub with a brush from now until the turn of the next millennium and it wont come off!!
* Take care with the cappings on the end of the roof spars just above the gutters. They can be loose and come off very easily. You might even find that they are broken if it's been cleaned before by someone who hasn't been as careful as you'd like.