I was thinking earlier that I've left the painting outside for another week, with no extreme weather and no results whatsoever. I was getting frustrated, and suddenly it started raining!
It's going quite hard, and for the past week the canvas has been placed flat on the ground rather than propped against the wall, so that rain collects rather than drains off, and sunlight should shine directly on the surface.
This must be the first time I've ever hoped for rain and bad weather, and now I'm very pleased. Really looking forward to seeing the results of this, and hopefully it'll be raining all night!