I'm still sketching from the studio, but treated myself to an easier time - this is the view from the regular window at the side of the house: no acrobatics necessary.
I had to do it over two days. My new idea (while waiting to get my hands on some better waterproof ink) was to try laying in some colour first, adding the ink on top, once it was dry. I tried to be intuitive and do it without pencil guidelines, but all the roofy perspective was more tricky than I anticipated and, by the time I'd finished the watercolour bit, the light was nearly gone.
I finished it off the next day at roughly the same time in the evening.
It worked OK, but far too much faffing about. Anyway, I now have another ink to try: Higgins Black Magic Indian Ink. Ridiculously expensive: £5.35 for a tiny 1fl oz bottle, but recommended by Seamus Heffernan in my Illustrated Life book as a non-clogger - we'll see!