Always sub contracted out the window cleaning to a professional company, very often without a handling charge. My thought was the more services I offered that customer with the least amount of hassle the more likely they were to keep using me. Don't forget any complaints will come to you and you'll have to deal with them, just make sure the window cleaner understands it's your contract and they invoice you, you will pay the window cleaner, the company is charged for the windows via you. In time they will probably get to know the window cleaner and ask what he is charging anyway, if you are charging the same then neither have the incentive to hop ship. Not great business sense I know, but you're making money from the cleaning anyway and for the extra few quid you could charge is it worth it? On the whole you'll find most window cleaners are an honourable bunch anyway and when you find a good one that turns up when he says and does a good job he knows he'll get more work from you.
I think its all about gaining trust with the client, in time you could get other extra work and services and charge extra for those. Does that make sense?