Thanks for the comment JO. It's motivating, specially as I realise all the shadows need to go and be redone. I had the Sun's altitude all wrong.

Also, I've read about your technique of drawing parallel lines to the light direction. I started with that and then came up with a better one (although slower to process). I resized the lightray drawing so that the circle has a radius equal to the shadow produced by a one-story tall building (roughly at 3m high). So, the entire circle covers the shadow of a 6m pole.
Having done that I then
- move the lightray about everytime I do a new building
- placing the intersection of the circle and the lightray at the tall points of the building (4 corners, roof vertex)
- marking with a point the extent of the shadows cast by a pole at that point (on a separate layer)
- and then tracing the area that doesn't "see" the sun

This is my geeky-lengthy-unhealthy way of doing things... but, you know, that's how they teach us at the Monastery