Speedo head is a doodle to replace,
The dealer maybe right about it being the head.....they might also be wrong too, so check the fuses and it's worth checking the multi plug on the Vehicle speed sensor....thats the one at the end of the gearbox on the right hand side.
Remove the battery negertive
remove the steering column covers
undo the 4 nuts which hold the steering shaft to the dash
unclip the indicator and wiper switches
remove the 4 screws from the binnicle trim panel
remove the 4 screws from the binnicle and disconnect the 2 multi plugs on the rear of it.
By now the binnicle is out
Pull off the trip reset button
The 2 ½s of the binncle are clipped together, press and prize them apart taking care not to scratch the clear section or damage the lugs!
Undo the torx headed screws around the speedo...some also hold the fuel and temp gauges and then pull the speedo head free from it's electrical contacts
Build up is a reverse really.
The speedo is a static sensitive device so be carefull what you touch.
Should take around 0.7 hours and the last time i did one the head was approx 125 quid +vat
Hope that helps