I don't think it's just the amount of miles your car has done, it's the age. I mean, its a glorified rubber band hurtling round a set of pulleys bending this way and the that, being stretched and released as revs fall and rise, in an environment that isn't conducive to longevity (for rubber anyway). I think the book say 72000 miles or four years (however shoot me down in flames if I'm wrong) and looking at the age of your car that suggests that it might just be worth doing. I also understand that it's worth changing the water pump at the same time as it tends to be this that causes the catastrophic belt failure rather than the belt giving way of it's own accord.
IMHO I'd get it done (My 159 has done 18000 miles, is three years old - nearly - and I plan on getting the belt changed next year.
Also have a look at some of the independents on here see what they recommend and that will give you an idea of what to expect.
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........