get yourself a pole,
some people use, tfr traffic film remover non caustic, and spray it on using a hand pressure sprayer, you can leave this ten mins to work its magic,
or my personal favourite, "powerfull" hard surface cleaner from janotorial supplies, but dont leave it on the glass too long i.e long that 1min will stain the glass, got to work in small areas and rinse it off, but cuts through everything, i use quite a strong dilution i.e 1part powerfull 8part water
and then a brush from unger the high flow one, cheapest place is soapnational.co.uk and a unger or pulex pole, and some scrubbing action,