David,
If you're desperate because you're cash flow is naff and the bills are coming, try this:
Friday evening knock to collect, and if they're not in, leave a note saying:
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Your window cleaner has called to collect but you were not in.
I will call back tomorrow morning (Saturday) between 9.30 am and midday. If you will not be in, or do not wish to be disturbed, please leave payment under an object on your doorstep. Cheques are to made payable to Mr D Coats.
We are a small business and our cash flow is important to us; we thank you for your co-operation.
Contact Details:
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If when you call back no one is in, and no payment is left out, leave a SAE with payment instructions.
So in effect you collect Friday and if they're not in, you leave a note saying you're going to collect in the morning. And if no payment is left, then you leave an envelope.
This isn't ideal, but if you need the money, you get your ass out there and collect it.