2002 astra 1.7dti speedo not workin?

the speedo on my car has stopped workin and no-one seems to know whats up mechanic says its electrical, autoelectrician says its mechanical. the milage does`nt move but revs and all other dials work engine management light comes on after exactly 5 mins of startin i think the car has had a new gear box but have had the sensor checked. "possibly the hub sensor" i been told but dont wanna spend 300 unless i`m sure any idea`s ????