Thanks. For that second shot I was initially using multiple points - I can't remember if it would have been zone AF or expansion, but the camera was floundering in the poor light (that's shot at 3200 ISO) and the only way to get the camera locking on was to remove the uncertainty for the camera by switching to single point AF.
I've also read something from Canon, in reference to the 1D3, cautioning against using expanded AF points if you don't need them due to the extra data generated, which all needs analysis by the camera before it can work out where to focus next.
Combining personal experience with that titbit of information I have since adopted the approach of using the fewest AF points necessary to get the job done.