In reply to forcan: We walked in a bit later, and although it tipped it down during the walk in, it started to improve when we hit the col. So we caught the better weather later on. The freezing level dropped sufficiently quickly to give reasonable conditions, and the sun even came out for a bit!
Some of the slopes were looking dodgy (some weakly bonded layers) so we stuck to the far left of the coire (on the west facing slopes) and did a short easy climb on a buttress not in the guidebook. Everything was fully frozen at that point. Walked off the back of the hill down to Loch Muick - good day out really, even if not what we originally planned.