It's a bit of a black box of a rig...there are set deiven keys, constraints, and possibly switching attributes that are buried.
I tried being really careful with name-spaces, and went round the houses a couple of times and Atom wasn't solving well.
I managed to do the transfer with studio library.
It's great. It can manage working across name spaces, copying anim, chaining anim and has a built in pose library function...10/10 will probably become a tool I lean on in the future.
https://www.youtube.com/results?search_query=studio+library+maya+tutorialthis video specifically
https://www.youtube.com/watch?v=N_Al1hXK_RThanks for the input Anthony and Stephen, appreciate people being around