Mark,
Go take a look at
www.mgcltd.co.uk they sell a biocide product called "Lichenite" to clean roofs, (its what the councils use, and lasts quite a few years) just spray on and leave for 72 hours, wash off with low pressure wide fan pressure washer (always point pressure gun in the same direction of tile, ie downwards towards the gutter at a 45 degree angle, otherwise you may lift tiles).
Check the weather forecasts, (you don`t want it to rain within 72 hours, or you will have to do it again)
The witch picker thingy is a good tool to use if you are cleaning bungalow roofs, but not much use for two story buildings, its then best to hire a cherry picker with operator.
Make sure you clean the gutters afterwards.
They also sell various roof sealing products like "Haloseal", some which can be sprayed on if the roof is damp.
Once cleaned, this should last the customer 5 years minimum.
You should look to make a minimum profit of around £350.
Make sure your insurance covers you for this type of work!