Argh. Trying to tighten the belt, I removed it, re-taped it, and re-installed.
I then tried a sample print without re-setting the Z maximum. Doh! The print head was pressing against the steel surface, allowing no plastic to be extruded.
I noticed after a minute or so, and stopped the build. X, Y and Z axis all move, but the extruder doesn’t respond. Crud.