Dave,
I have almost no experience at employing window cleaners, though I did it briefly, once; he lasted two-weeks before I sacked him because he couldn't turn up to work on time, but I'm guessing that you suspect your lads are taking too long to do jobs because they're taking too many breaks; though I could be wrong.
However, the way the army do it is fairly good. They pay a 'day rate', so no matter how hard or how little they work, they get paid the same day rate; but obviously they've got to do what-ever tasks are set them; and if they've got to work longer than usual some days, so be it.
So if you want to pay your guys £60 a day per man (for example), you set them an achievable amount of work, and let them work out when and how they take their breaks; and if they finish early - given that all the work is done to the required standard - they get their £60 a day.
I think this method of working is called 'Spanish Working Hours'; but I guess it does have its pitfalls too. It could encourage them to rush, so they can finish early, so the standard of work could be lowered. But to prevent that, you'd have to be out checking up on them; showing some leadership and management stuff!
I hope I've been some help; but I doubt it.