water and a brush is not the most hi-tec cleaning agent,pure water and a brush is not much better(IF AT ALL).
A good non-caustic tfr or similar solution and a brush will take off most green and black contamination from a conservatory.
If someone is requesting a conservatory roof clean,in my experience they are after a deeper clean than just water will give.
Pure water is not a miracle cleaner by any means,it probably cleans just the same as your tap water would,its only advantage is that once effective rinsing has taken place it will evaporate without leaving residue.
It is very very easy to believe that pure water has some magical cleaning ability,this simply is not the case.
I have a nice wfp set up but if it comes to a dirty conservatory then i would set about it with mine or the customers standard hose ,cleaning solution sprayed or if the sprayer has ceased to operate again then the brush on the end of any pole(unger optiloc in my case) dipped into the bucket of cleaning solution(yep,fairy liquid etc will suffice).Let soak,agitate,rinse off with hose gun on jet,agitate again for the stubborn bits and those bits trapped in awkward crevices and blast off again with the hose gun. Try and blast away from edges so you are not testing the seals as they may probably only be rainproof if the conservatory is older or not too well put together.
At this stage final rinsing with wfp ought to see the roof panels dry streak and spot free and save having to attach a sguegee and applicator to the end of the pole which depending on the access can be a right pain in the arse and worth avoiding altogether in strong direct sunlight.
Good luck,invest in a good cleaning solution and go at it with confidence and remember that pure water IS just water.
