Hi Ben,
After searching a bit I found the examples above which was not what I was expecting. The whole table (there were many more HC's in it) showed that the density and octane rating were pretty much unrelated.
I couldn't find anything on the volatility aspect so I can't do anything but assume, in a sort of random guessing type of way, that the lighter parts are also possibly more volatile. Maybe. I suppose.
Anyway, the way I see it now is that stored fuel loses some of its lighter parts which reduces its willingness to bring your engine to life (carefully avoiding the words pre-ignition and detonation here). This also has the side effect of reducing the octane rating, although I'm now sure it would be nowhere near 1% per week.
I would expect it to be at most maybe 5% in total once the lighter high octane parts have gone. This would also explain why old fuel will run without ruining your engine - the octane loss just cannot be that big. It certainly takes place but isn't the cause of the problem as I now see it. It seems to be just a side effect.
Bye
PS Ethanol has a RON of 126. This means I have some fairly high octane "fuel" from Scotland in my posession!