Re-targeting mannequin animation to new skeleton 'collapses' mesh?

Hi There

I am trying to reuse some of the ‘free’ animations (ThirdPersonWalk) on a custom skeleton (that is identical to the mannequin skeleton except the twist bones + the A pose is slightly different).
I can run my own animations and they look fine but when I try and retarget an existing mannequin animation the mesh ‘collapses’ (see ss). I have scaled the imported skeleton x100 to make it fit UE so I suspect something goes wrong with the weighting?

Can anyone give me any advice on how to fix this or point me in (any) direction?