Brymak, the fibre optic bulb unit from what I can remember is under the dashboard instrument cluster. On the mk1 Uno, you have to take out the instrument pod first to get to it. Once the instrument pod is out of the way you should see where the fibre optic wires lead too. Also from what I can remember it's a bit of a pig because it's held in by a screw from underneath that makes it awkward to undo. Other than that it's quite simple to change.
Hope this helps.
Chas