after 10 years of business and trying lots of different brushes I find the best is the flocked medium sill brush from Gardiners. Its nice and light and if you get the angle right it covers sills, frames and glass. the only thing about the medium is that it isn't great for bird poo and takes a good soak and scrub to get rid of it. Saying that, I could just superglue a scourer to the stock end of the brush. Been meaning to do that for ages but never tried it. Job for this B/H!!!
BTW I'm on my 4th consecutive flocked medium sill brush.