In reply to nicnic:
Thank you for all for your replies. And for correcting my misunderstanding about the definition of bothy.
We ended up staying at the Corrour Bothy on Saturday night, as we met another guy who was heading up to the Garbh Choire shelter, and we thought 3 might be a squash. Although that was before we realised we'd be 8 at Corrour. And before I'd read the above about 11 having once squeezed in at Garbh Choire!
We did go up to have a look at it the following day, and we spoke to our friend who stayed the night. The door is off the hinges and there was some snow in there, and signs of gaps. The guy who stayed there said it was very breezy and some snow got in overnight. But as others said previously, I think you'd be ok with a decent sleeping bag and a bivvi. A stunning place to be of course if the cloud is high enough.
We also bumped into the Cairngorm Wanderer before we got there, wandering about in the Cairngorms, appropriately enough. I asked about whether there had been any movement on the landowner allowing the shelter to be repaired and he said nothing has changed since that 2014 post on his blog. Essentially, he said, the owners believe that having a shelter there detracts from its status as one of the wildest spots in Britain. However the response to that is that the main way it detracts is in the make shift ways people have tried to block the gaps (plastic sheets, bits of tent), and that a proper renovation would deal with that.
I'd also like to say a big thank you to those who renovated the Hutchinson's bothy and put in the fire. And left the fairy lights!