I agree with Malc Gold. Not only does he make you £40 but he does £100 worth of work you don't have to do.
If you want him to work longer to earn you more, then he might not be happy or his productivity might drop.
I'd try to keep him sweet; maybe offer a higher rate for longer hours; talk to him about it.
Good window cleaning staff are not always easy to find and I'd be happy with someone doing 60/40, if only for 4-5 hours.
It's enough trad, anyway.