Speedo Issues Mk1 Auto

Hi All

My speedo is playing up, in that regardless of the speed i'm doing, it only goes upto 30-40mph. The mileometer is also moving at i guess the rate of the speedo, as i did a 120 mile trip and the mileometer said i did approx 50 miles
Ive been told its not the speedo cable as it would either work or not, so must be the dashboard itself.
Any ideas where or who could fix this issue?

many thanks