roof cleaning

I live in aberdeen,can anybody could be the roughist idea what would be a fair price to be expected to pay for the brushing down moss etc from roof and conservatory.I live in a bungalow with a slight sloping of roof on the conservatroy which has collected the rubbish blowing from the roof. What do I ask about what is involved also if I ask if they are insured what if they say yes how do I know?I wouldnt want to claim on my insurance if there was any damage,anything else I need to be aware of before I get quotes thank you
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