Well I haven't got the written reference to hand, but I am sure that you should not be deducted by these rediculous percentages unless you are part of the construction industry. Which, as a window cleaner you are not, so no exemption 715 certificate is required by you. If they cannot produce written proof of regulations pertaining specifically to post-build cleaning, after which, Build Regulations are lifted, then they are making their own rules. So as has been said, make your own prices. Add- accordingly to your prices.
Unfortunately, closing the door after the horse has bolted, I fear.
Pj