Masking animation without avatar mask

I've managed to create a scene in Unity 3d where I've imported a model (with no animation) into the scene. I've then applied some animation (from an FBX file - which just alters the parent in the hierarchy nothing else) as well as a script that alters some of the animation as well (higher up in the hierarchy).

The scene works fine in that I'm able to see BOTH the scripted animation AND the pre-made animation work together.

The bad news is I have tried to re-recreate this with another model and can't get both animations working together. Either the imported animation overrides the scripted one or visa-versa. Here's another thing I'm pretty sure I wasn't using an avatar mask either.

This is a long shot...Any ideas where I'm going wrong? - or probably easier, how to import animation onto a model as well as a script that created addional animation and get to see them both working at the same time.
Answers on a postcard please...