Okay, the cake has been baked and eaten. As far as I could tell, it didn't
suffer the flour-mixing problem after all. I don't know why, since it did every
other time I made it. It was a nice cake, but it would have been better if I'd
baked it for 5 minutes less. People finished it off pretty quickly. The icing
ended up being the biggest problem - too hard, too much cocoa and it had become
discoloured around the edge by the next day (eating day).