Mmmm.........I give up Rog, how can you clean what you can't see?
Ok, seriously, of course you can't actully see the sill, but it really isn't much of a problem to clean it, particularly UPVC.
Wash the winodws and then give a light wipe with the brush.
I really don't have any problems at all with them.
You can't put the pressure on that you could if you were using your washer or a scrim, but lets face it, most of us give them a quick squeegee off or a cursery wipe with the scrim you use to mop up with.
Now some of you may well spend time burnishing every sill to a high gloss, but most just wipe the excess water from squeegee-ing off the glass.
Doing georgian or leaded windows??
Half the time I'll bet you just give the sill a bit of a rub over with the scrim you are cleaning the window with.
Not many will wet the sill, wash it and then wipe it dry with a clean scrim, I sure as hell never did, and as Squeaks and anyone else who has ever worked for me will testify, I've always been particular about making sure sills are cleaned.
They just don't take much effort, and the same goes for WFP, by and large you'll do just as good a job as you would trad.
Though because you will have done a good job on cleaning the frames, the sills may now stand out if you haven't made the effort to get them right on the first clean
I've often thought about attaching one of those pads on to the brush at some point too....with bird poo and so on being the reason....haven't done it yet though!!
Ian