I have exactly the same problem on the same age Zafira 1.8. Fuses are all fine, so I have come to the conclusion that it is a backlight bulb. When I have worked up the courage I will try this:
http://marty.sunriseroad.net/2007/11/22/replacing-an-instrument-panel-bulb/
and will let you know how I got on.
I assume your speedo is working fine, so with any luck (and a bit of skill) you should be able to replace the bulb yourself for a couple of quid and an hour or three of your time.
Simon
October 2009