Welcome to the forum, Graham.
The "problem" is the gap definition. You have a zero gap on the side and the top and to do this right, it has to have the transition through that bend.
To do what you are asking for means you would have a small gap on the top flanges (the 45 miter) and no gap on the vertial edges.
So as Andrew said, "unbend" the part and miter the edge and then "bend back".
There might be more methods but from what I know today, it requires more than a single step.
