No problem Giles,
I make 3D audio since 1997 and I am used to this kind of misunderstanding. The three basic faders for Ambient Simulator are x, y and z of the audio source in input. Those refer exactly as you say at left-right, back-front and down-up. The 5.1 is practically 2D but you can add another AmbSim and use Spider to send some copies of your source also to the second one. The first will take care of the low level of speakers and the second of the upper set of speakers. Choose how much copies of the source you want to send and realize the desired setup of speakers, any combination works just because you have separated inputs. From 8.0 to 10.2, from 4.2 to 9.1 (the one I prefer, you have stereo, 5.1 and 8.0 in the same setup, working also on a normal 7.1 home theater if you add two satellites and a switch). Obviously the xyz data sent to both the REs must be the same. I am working at an update that will also correct the delays for the upper side, but it is just for fine tuning, in little spaces like the ones address by the traditional surround home theaters the approach I described works already very well.
Do not hesitate to write again if I have not been exhaustive or you have more questions, thanks again and
All the best