Sovled
Just as a bit of a follow up to this, incase anyone searches for it.
The MH 3512 was the correct changer, however it also required the wiring harness (available from Command Online or the dealer [£47 with command])
Fitting the harness is the hardest part of the retrofit as it requires both the driver and passenger side carpets to be removed (as well as the gear gaitor, ashtray, mechanical drawer and head unit)
Your car may also require coding for the autochanger to work, fortunatley mine didn't as I had the phone installed already.
Took me around 3 hours to, but this was more due to removing trim etc than the techincality of the job.
Overall though very happy and a worthwhile upgrade
