after a couple or three cleans who cares :
i use vikan sill and oval
bentley(superlite)
salmon
and various diy jobs
if they have been done regular a bloody masonry brush will do the job
christ i could just spray the glass on 80% of em and scrub the odd bird turd and no one would notice 
Bird turn and bentley is asking for 20 times longer min to get it off, it could well be me but the birds are dive bombing a lot of the windows we clean at the mo, and I swear they have cement in there poop bentley I binned as took far to long to get it off.