I worked it out pretty late last night, it was nothing to do with unreal. It was actually all caused by 3dsMax doing some strange things with the scales or something - even though they all appeared fine in both max and unreal.
I had reset X-form across all the meshs multiple times trying to work out the issue, so assumed it was all good. But I found that If I reset the Xform after having linked them to the body, the scales of everything went off drastically, as though the Xform hadn’t corrected before. I then re scaled and assembled the model again, and this time everything worked great.
I think Maxs units had all gone wrong too. They were all set to cm, but the tape measures readings were nothing like in the right figures.
So with the Xforms reset and exporting from a fresh scene everything was working fine. The plus side is that I am now much more knowledgeable about setting up models in Unreal, haha!
Thanks again for your help anyway 