Don't use customers tap water because you will get hard water all over their windows.
Work out a price per metre you're happy with, and go with that. Add extra £50 here and there if they're filthy, high, difficult etc.
Talk up the job to the customer 'ooh it's a nice house, lovely size, it will look so much better better with all your PVC gleaming, we'll even do the windows and garage door in the price for you' (They're going to get wet and dirty anyway, plus you add the cost onto your price). Sell it as a specialist service - they're not going to go up a ladder themselves and clean by hand. Do a test patch to show them the difference.
Some you'll be surprised how much you can a charge and get the job, others will expect a £30 job. Don't be annoyed if you don't get all of them. You're better off doing 2 jobs than 3 and still earning the same money.
Thanks
Tom