Malc,
Give Lime Lite a go. It`s fairly cheap and available at Tesco.
It`s a limescale remover but also gets rid of soap scum and slows the build up of dirt so I would think it would be ok for future wfp cleans.
I`ve been on my 2nd cleans today since I first used it and they were much easier than usual despite strong winds again.
I do add a some Ecover as well for a few suds as I believe this doesn`t leave a residue.
Gerry.