Check the cable has not just popped out of ethier end first,if not the cable is more fiddley than hard to replace,if that is the problem [?]
It should be located into the gearbox by a single screw/bolt,then into the back of the speed by a press clip IIRC.
When it off it should be quite easty to see for sure its (the cable)snapped.
TBH i would be surprised if the garage would have to touch the speedo cable (depending on its route) and its possible it was just reached the end of its working life at the wrong time.
"POSITIVE WAVES ARE WHATS NEEDED"