That all went better than I feared.
After the effort involed in making holes in the steel pieces, drilling the wood and brick was easy. I chipped away the render so the braces were lying flat against the brick, pushed the bolt through, and did up the nut. And that was that! (Well - almost - it rained while I was working outside, and I will need to wait for someone to arrive with a couple of large shifting spanners so I can get that extra last 1/2 turn with someone up the ladder holding the bolt still)
The render is chipped away...I was so impressed with myself that I even managed to screw the saddles to the bearer.
NOW I am confident the floor isn't going to go anywhere unfortunate while someone is in the shower. Even without the bolts being fully tight the bearer is solid.
What the braces look like from outsidePleased? You bet I am. In fact - I am right chuffed.