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)
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.