Tom,
As has been said uPVC solvent cleaner is the stuff you are looking for but although it does a great job be warned it won't always remove everything, as long as the customer understands this you shouldn't have a problem.
As Daniel says £80 does seem very very cheap for this service, that is nearer the price for a uPVC wash down.
Remember you will have to wash down the uPVC to get rid of the grime, then get up there and apply the solvent cleaner by hand which can take some scrubbing then rinse it all off again.
To give you an example I did one last week on a bungalow it took 6 man hours to complete and we charged her £200 and that is 2 years after we last did it so the uPVC wasn't that bad at all!
Daniel,
Your job sheet sounds like a great idea, if your feeling generous would there be any chance I could take a look at yours.
alistairstott@hotmail.com Regards
Al