On my own, I've accomplished a small bit of diagnosis. I've identified the leads from the proximity sensors for the x and y axes. Swapping them around changes nothing in regards to which is illuminated, leading me to believe that the sensor is not at fault.

Another aspect of this recent failure is that the machine apparently thinks that it is a 300 x 500 mm unit, rather than a 500 x 700 mm unit. I suggest this is due to having used the factory default option from the panel menu. I believe the 300 x 500 limit figure is correct because the y axis moves only to an indicated 300 mm distance on the bed, leaving the lower portion inaccessible.

Does anyone know how to modify these limits?

Also I have discovered that the diagnosis menu shows a red flag in the -x limit box, which is unsurprising and more-or-less confirms my understanding of the failure.