ha, ignore me.

I have solved my problem. Both axis had software min endstops on. They were inverted. This was not the end of the problems.

The PLA had degraded or got damaged on the x-axis and the x-axis endstop had fallen behind the zipties that hold the belt in place. Tightening the screws would have been enough but the PLA just crumbled.

The y-axis was behaving the same, but it was due to a cable being in the way. D'oh.

The z-axis wasn't triggering because a wire had come loose.

When my friend tidied up the wiring (he's neat like that) he'd rewired the bed to the hot end and the hot end to the bed...

Fun times.