Almost there now. Stepping back to make sure I hadn't compromised any proportions, which might result in a contrived-looking scene, I saw that there were greens in the greys and yellow in the whites, so I added these, and suddenly the painting was 'lifted' to another level. I'm definitely going to do more sky gazing, making a note of any colours I see other than blue, grey and white, for future reference.
I built up the depth of oil paint in the lower portion, as I knew I wanted to slice into the paint when adding the rooftops. This is when I started to hesitate. How to add the rooftops. I only had a fleeting urge to leave this one as it was, and add rooftops top the next painting. But NO! They were an integral part of my idea. I must be brave.
You'll have to wait until tomorrow to see if I pull it off. here's hoping.