Ok Ill admit im very late round this time - but in a week had 2, yes two say, not today thankyou?!
Never had this before? Any ideas? Makes me feel bad for some reason 
Pretty much from the word go, with a bit of common sense, I've always dropped customers who say "Not today, mate!" It's unthinking, it's rude, and I always explain my terms and conditions when I quote.
Consequentially, when I get a 'Not today, mate', it's very rare, but when it happens I just remind my customer of our terms and conditions and explain that if I don't clean 'em today, I will take them off my books. Sometimes they go "Fine!" and I shrug my shoulders and walk off, sometimes they go "Fine" and later come back and say, "Okay then, clean them", and sometimes they just say "Oh, you'd better clean them then!".
But I'm not overly strict; a customer's husband died recently, she explained that to me at the door and after a chat about it I asked, "Do you want me to leave your windows this month?" Or if they've building work, or if they're about to get their windows replaced, or even if they're skint.
But it does annoy me to pull up to a property, start dragging hoses out and then a window opens and you hear a "Not today, mate!"