I’m having some issues with the speedo in the Elise, it regularly flickers below 30mph bouncing between 0 and 30, also it can fail to work at all.
The car appears to have a new speed sensor fitted to the nearside rear hub, but the ring on the drive shaft is very rusty. Is it worth cleaning that up? Try a new speed sensor or start trying to hunt down a broken wire?
If the ring is very rusty then do give it a clean first. Also check any connectors for dirt, corrosion, etc. A broken wire would be more likely to cause either (a) no signal at all or (b) erratic readings right across the range, rather than primarily lower speeds.
Also make sure that the sensor is as close to the hub ring as possible ie. almost touching. Mine did the same thing, erratic below 30 and over 140, moving the sensor sorted it.