I went for a different approach today: having decided the Post Office was going to be whitewashed, I managed to find a few minutes when the sun was out and it wasn't windy to spray it white, and the church grey, prior to assembling them. Here I actually encountered the first problem with the Warbases cutting - the church tower has slots to take the flat tower roof, and those for the small church tower piece that faces the rest of the church roof were marginally too thin. I actually had to file the slots larger to get it to fit.
I went for the same approach as with the red brickwork with the grey stone - a thin but generous wash of dilute Army Painter Skeleton Bone. Seems to have worked!
For the Post Office I'm considering a similar wash but of a very slightly off-white, as if you look at the typical whitewashed building you can only just make out the mortar courses.
I'm also debating a different approach to the windows. Watch this space :D