With any job you have to pay the minimum wage, but you can word it to say it pays in line with he minimum wage act and still only pay on commission only basis.
What you do is advertise for a part time canvasser for 16 hours per week, you then pay them a say £10 for every job the get you, this charge is added to the 1st clean price, you then deduct this from the minimum wage you have to pay them.
So if you take someone on at £6.08 per hour, you have to pay them £97.28 for 16 hours work, but they will have to be a numbly not to get no work from 16 hours of door knocking, if they are, get rid and replace.
Also take them out for a work trial before you start them, if they get business on this then they will get business if you are paying a wage