Simon, I understand your problem as I have a lot of those type of windows in my round. If I'm totally honest, I'd say this. It depends who the customer is and how good a payer they are. I have gone back to trad on some houses because of this. I feel satisfied with the job and there is no faffing around, but they are properties I'm happy putting a ladder to. If you have to wfp them I'd say, don't get too into the spiders' nest and all the rest that's hiding under the sash window. Clean the glass and leave it at that.