There's a chemical am using on a glass roof, i started off with virosol and it didn't do enough, now using there hd cleaner ( it's blue coloured ) which is doing he job, supplier is ics and am sure there nation wide so would be one local to you
if you got a cherry picker, just put a bottle into a bucket and scrub straight on, water coming out of brush will dilute, but were we have really bad glass I have pre wet and brushed it with a dry brush to work chemical into glass,
Best to ask supplier if it will leave marks on the building were it runs down