I've cleaned my own windows loads of times, sometimes good other times bad with spotting, think I've sussed the problem. Theyre upvc and the spotting always occurs on a window below a little opening window, where i've been scrubbing the frame bristles or jets of water must be getting slightly under and drawing  dung out , as i opened window dfrom inside and they get pretty mucky. cleaned them last night in the rain glancing over that frame and this morning no spotting on any of windows