Cracks me up, cos the ones that are knocking me for it, had they of been in the same situation, would be £8 short, and have a bloke walking round the town thinking hes got the better of you.
No brainer really. 
Ok, try to see it from another perspective rather than just being aggressive about it.
It's nothing to do with it "only being £8" in the way that £8 is not a lot of money. For me, it's that rather than spending time on "endless phone calls, door banging etc..." and then time going round to collect, perhaps another approach would be to forget the £8 and use the time cleaning someone's windows who might turn out to be a good long-term customer. I wouldn't be "£8 short", I'd be "up one new customer."
My view is that my business is cleaning windows. I chase debts until the time spent collecting looks like becoming more than it's worth. I don't care what a non-customer thinks of me. If it costs more (in time, postage, driving, etc) to collect a debt than the value of that debt, I'd be mad to chase.
And now call me a gay, southern jessie who knows nothing about window cleaning and has only just started and has a smart van if it'll help you to deal with my disagreeing with you...
Vin