The tricky bit was removing the rear seat, mainly because I was looking at the instructions for the wrong model of car; the right instructions for me were on the Focus Fanatics forums. Fitting the actual bar itself was comparatively easy, just two bolts (after you’ve removed the bungs from the bolt holes), I’ve written a StackExchange answer. I bought the Isofix bracket and Torx driver set from Amazon.
The alarm in question looks like this:
The red light indicates a fault, which is why I’ve finally confronted my inability to remove it (hopefully your smoke alarm fault-free).