It appears the issue I’m having with my speedo is the same as everyone else. The fix is to cut off the ground wire on the speedo wiring harness plug (the plug that runs to the front of the car as part of the loom), then attach a new ground wire to the plug, and screw the terminal ring end of the new wire into the chassis crossmember, which requires drilling a hole in the crossmember. The instructions from Caterham in the tech note are clear and concise, it’s quite easy to fix although a bit of a pain in the ass to get the drill in there to make the hole.
After a quick 10 mile drive I’m happy to report it’s fixed.
[Edit: Nope, not fixed.]