This is a curiously unhelpful forum, is everyone trying to protect their jobs or hoping to make money out of tutorials?
I eventually found the answer to the above issue.
The solution seems obvious now, but the various ways I was trying to get this to work, didnt work until I found this. The reason I didnt find this, was because the start of my animation is slightly out at 0001 and when I was right clicking on body at 0000 there is no option to bake anything.
So, you bake your animation to a Metahuman control rig. Then the additive part works. I had also tried an FK control rig and though it animates in viewport, it wont render the animation, only the FK control rig additives. Lots of ways that seem like they should work, don’t.