Hi MetaTunes - there are times when I think that I actually have progressed in using the UI.
On the release page, over on the right hand side, “Editing” heading, click “Edit relationships”.
Next under the orange “Track Relationships”, you want tick all the required little square boxes. You want to add just plain-old “Simon Rattle” to all of the tracks? That top box will do them all.
Then click “Batch-add a relationship to recordings” and enter Simmo the way you want him.
Then go through each track and mark for removal the "Sir Simon"s.
Then down the bottom click “Enter edit”.
I’m sure you knew that last bit but once upon a time I did sit and wonder about how to actually proceed on that page.