Just got off the phone from her and this is how the conversation roughly went:
ME:
Hello Mrs Smith, I've been thinking about our working relationship and to be honest I'm not over-happy with the other guy coming to give your windows a quick once over with a damp rag when he needs a few quid and I'm the reserve for special occasions. I don't have a problem with quarterly cleans as long as the customer sticks to the agreement and it is 4 times a year...this way I know roughly what my income will be and I can budget my life accordingly. With ad hoc cleans I'll never know from one month to the next what my income will be.
HER:
I hadn't thought of it that way, I just assumed all you guys had work coming out of your ears and turned up as and when it suited you.
ME:
Well obviously that's how your other guy does business...but I don't. You want quarterly, that's what you get with me. So please choose him or me.
HER: Errr Ummm Errr...I need to think before I can answer :

ME:
Don't worry, you've answered...BYE!
.........and another one bites the dust!
Better get my canvassing skills up-to-date at this rate
