Hi Kev, I have a similar problem with my 2007 9 5 sport combi . Now and then the tailgate door refuses to open using key fob or drivers door side switch.Update: Gained access to the boot via the back seats, and checked out fuse 21 in the rear panel (control module in rear doors), but it's fine (which I knew anyway as it's the same fuse for the parking assist). As far as I can tell there's no manual/emergency rear door release on sportwagons/combis, and it's not reacting to the buttons on the fob.
In desperate need of help here ...
I have found that starting the car sometimes fixes it. Battery tests good, so it is still a mystery. Only other thing I have found is to keep latch clean and free of debris.