So...
It wasn't my dealer than sent the paperwork to the DVLA to have the cherished plate removed, it was the BMW garage where the previous owner traded it in. Long story short they filled in the paperwork incorrectly and it all got returned to them. So I've waited 3 weeks for literally nothing!
Waiting to find out on Monday whether they'll let me take the car on the cherished plate anyway for the time being, and transfer the plate over to the original once BMW and the DVLA sort their sh!t out. Why this couldn't be done 3 weeks ago I have literally no idea.
Obviously I'm expecting them to come back and say no, you have to wait.