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Ptown
Since posting my question I have been led, via the wikipedia article on Q-Factor, to Disley and Li's interesting paper on self selected Q. In their study they found that SSQ leads to higher knee stability and higher mechanical efficiency, and also that SSQ could be reliably predicted using a simple suspension test.
Taking the advice to heart, I tried suspending myself between two office chairs while I was waiting for the kettle to boil. I eyeballed my inter-medial-malleoli-distance to be around 70mm, and that using Disley & Li's equation(1) my SSQ should therefore be approximately 154.57mm.
So now I have 2 new questions...
(a) @xavierdisley , It's not clear to me from my reading of your paper how I should translate my estimated SSQ into a real Q factor. In lay terms, Q seems to be measured as the distance between the outer faces of the crank arm (e.g. here), however this wouldn't account for differences in Q from different pedals or cleat positions (which varied between participants). When you were measuring Q, where were you measuring it from-to?
(b) Since my pedal Q is fixed (I'm committed to vector pedals) is there a useful database of Q factors for different cranks? It's a faff going through each manufacturers specification page individually...
(1) y = 0.569x + 114.74