Quick tip for anyone who stumbles across this:
I found if you have just purchased some animations, they will come with their own skeleton.
When you add this to your project, you never get a chance to target your existing skeleton.
The way I got around this was to simply delete the marketplace assets and replace the reference with your existing asset.
once you have done this for the skeleton, phys asset and mesh (I did it in that order) fix up redirectors and you’re done.
You may also want to delete folders the engine may have left behind after the deletion in your file explorer of choice.
You may also need to restart the editor after such a heavy operation. I had mine crash (Epic, a bug was filed for this)
Hope this helps anyone.