For conservatoies I use a Bently brush because it's so light.
The problem is in cleaning the ridge cappings (fancy, fiddly bits) and finials (pointy bits) which can be quite loose. I found that a heavy brush like the Vikan had a mind of its own and made the pole difficult to control. This is because you're working at such a low angle. The Bently brush, being so much lighter, was much easier in comparison.
The secret of success is to soak them with tfr and, if the soiling is heavy, work it in well. Leave it to soak in for just two or three minutes. The gunge just floats off then with the minimum of scrubbing.
And for conservatories with polycarbonate roofs, you don't even need pure. You can get away with tap water and then finish off the sides trad.