Would it not be easiest to put on a non experienced window cleaner, a decent fella looking for a job. Get him to learn the ropes doing bottoms only, or organising hoses etc if you are WFP. After about a week, he would be able to cover his wage at least in the speed increase, you can then take on those extra houses that you get asked to do.
Once he has got up to speed, teach him to use ladders(or better still employ someone with ladder experience in a different field perhaps). Then when he is right to go, send him on his own. He will still only need £10 an hour max and you can use some of your free time to go and get more business.
Find a people person to do the job and make part of his role to do some door knocking as well. Get a bloke with his own van too. Those aren't too much criteria to ask for when choosing someone, there are plenty of out of work plasterers, brickies, painters etc that would gladly take up a wage these days.