Kev I'm not ofay with C5, but if its liquid crystal, an educated guess is backlighting, you normally can not easily see LQ without a backlight, this most likely supplied from P'supply on main board. On your cluster you may find a little black hole or orifice that is the light sensor. This drives the display cluster lighting up and down depending upon ambient brightness. If so, get torch, turn on car, display lights up, blank sensor, display dims, shine torch in sensor, display should brighten up (max). If not then the dimming circuitry could be faulty, if all the cluster works except the odometre, then its Mr Google.
Sorry I cant be of more help.