You did say the milometer was working, assume you mean its clocking up the miles which would indicate the the probe is functioning and sending a pulse to the speedo, but as deejay says you can check the LED on the proximity probe to confirm that the probe is good. If it's the speedo unit you should send it to Caerbont (ex Smith Instruments), call Peter Wilkinson at Caerbont and plea your case for a fast turnaround, 01639 732200.
There is a wiring diagram on the Caerbont website for the speedo so you can check that you are sending 12v power to the probe.
I had all the palava when my 2011 Plus 4 Speedo expired.
