As suspected the switch had fallen over. Local restart (pulling and reseating the power cables) fixed the fault!
This has reminded me that I should put an Ethernet cable tester into my work tool bag.
@renbymon Just the tail sticking out of it
@noodlydragon Oh gosh yeah
"Who wired these cabinets?!"
@renbymon Be careful not to get eaten by the mimic server rack :D
Also I'd love to get artwork of myself working in a comms room, maybe arms deep in a mass of cat6, lol