Not all window cleaners can reach above 5 floors , so your rate must be higher than what you would normally charge.
Sorry John but I disagree,
My only reason is that at the moment you can reach 5 floors and are being asked to do the job. What happens if you charge over the going rate and all of a sudden they happen to meet a window cleaner who can also do 5 floors, if he is happy earning his hourly rate and quotes a lot cheaper than your price you will have a lot of explaining to do in order to keep the contract.
I am not saying your hourly rate is less or higher than the next but if you go in with the " I am the only one who can do it" approach you may find it back fires later on? If you earn £30 per hour then charge that and you won't be losing out and they won't be getting turned over.
Just my opinion and not necessarily the right one but I very rarely lose a job because of price.