A licence no way.
An inland revenue register of window cleaners OK.
If the tax man wants his cut from us all, then it's in his interest to have a register of all window cleaners. Give all of us a registration number that we can put on our windscreens, and on our correspondence.
Make it illegal to trade unregistered.
One registration number lets you trade in any area.
They bare the expense, and they reap the rewards.
We pay nothing but the taxes due.
What could be more simple than that?
Problem sorted. Dai