Even if you've a sign-written van, just stick a large note on your dashboard saying something like,
'Window Cleaner at work. Van will be moved by 10.00 am. Phone 070708909090 if there's any problems'.
Also, what about starting early? Traffic wardens don't seem to start work till around 9.00 am. I guess they'd get upset if you're constricting traffic at a rush hour time.