Ok, I'll put everyone who's been puzzling over what's different about the FR's latest 'superbarn' 108 out of their misery.
It's the handrails beside the vestibules.
For whatever reason, be it aesthetic, practical or just to save some cash, 108 only has them on one side of the doors whereas the previous carriages have them on both.
Here's the proof.
Goes to show that as a modeller you can't let your guard down for a moment - you've got to watch them like hawks.