If I were a customer, I would never agree to a direct debit for window cleaning. Standing order yes, direct debit, no.
I'd agree for a big company like british gas or whatever, but basically saying "I trust you to take money from my bank account" to a window cleaner? no way.
And the ever-so-amazing "direct debit guarantee" would not give me any comfort at all. I once got charged twice for my car insurance by direct debit, it was about £500 over. Despite the 'direct debit guarantee' it was a nightmare to get refunded - the bank simply didn't want to know and didn't see it as their problem.
Eventually I got the insurance company to refund the money, but it took months of wrangling and I never got the bank fees back for going overdrawn even though it was their fault.
I got the distinct impression that if they'd never acknowleddged their mistake (which they didn't for a long time) then there would simply be nothing I could have done.
Since then I'm extremely wary of what direct debits I allow, and only do it if there's absolutely no alternative.
But maybe thats just me.