For what its worth, the galley flat is in. I’m happy with the solution in the end, I agonised over it and put it off for a week or two, but in the end you just have to make it happen. My intended solution was to round the sides to make it easier to get in and take a sixteenth of an inch off both sides. It might have worked but I ended up taking an eighth of an inch off instead and then trying it. A Solution? The flat went in easily enough but I had maybe a quarter of an inch gap either side which I didnt fancy filling in with a fillet, so I used some 3/4″ quad, and it works and wont show since its underneath. Have now moved on.