Basically: -
If you are telling someone what to do and providing them with work, they are an
employee.
If they have to go out and find their own work, they are
self employed.
If they are paid a regular amount (per hour/day/salary) they are an
employee.
If they are paid per job, they are
self employed.
If you find people to cover work they can't do, they are an
employee.
If they have to find people to cover work they can't do, they're
self employed.
If you are supplying the main tools, machinery and materials to do the job they are an
employee.
If they are supplying the main tools, machinery and materials they are
self employed.
Bearing this in mind, it would be very difficult to find anyone other than another window cleaner to work for you on a self employed basis. As such, anyone you employ will be entitled to basic employment rights, even if you don't give them a contract, as the agreement of them to work for you and you to pay them is an implicit verbal contract in itself.
You can find loads of information at
http://www.adviceguide.org.uk/index/life/employment/contracts_of_employment.htm