Changeable, yes, you could certainly call South Wales (or indeed British) weather that, it does seem more changeable than most places it has to be said.
Hope you're right about the forecast though, it would be nice to ride this event in really nice weather, although if it's too warm I guess hydration could be an issue, there is not exactly a lot of shade on this event, unlike the Forest of Dean I did a month or so ago, so sun protection will be a requirement (I turn like a lobster in even 18 C weather).
Completely mad number of people doing this event though. What was strange last year was that I rode most of the event hardly ever seeing anyone (well once we'd gone up the Bwlch anyway).