Today I decided to bite the bullet and remove the lorry strap holding down the wall sill.
I started by tightening the four fixings. I was worried that they wouldn’t be sufficiently fixed as, on one of them, I had a problem with the epoxy resin running out of the fixing hole into the rest of the wall. But they all took the tension well. None of them were loose.
I then removed the lorry straps, et voila!
Another small but significant step forward.