It does sound as though the speedo has had it.
IIRC Caterham have changed the speedos (on the newer cars) which means you will need a speedo a cable and a speedo drive!
Double check with them.
They can be repaired. If you want to do that I can dig out a number for you.
How about an ad in the wanted section? There must be a few cars which have a Stack speedo in.
The heater and Instrument fuse powers the heater and the gauges ie the fuel and temp gauge. The illumination for the gauges is on the same circuit as one of the side lights.
James