I would've expected about £550 for a service & cambelt change from a main dealer. I paid £700 but that included major service & cambelt, waterpump, a suspension item, brake fluid change, clutch being bled & a sidelight bulb that requires removal of the front bumper (on a Stilo). £788 seems very steep, especially considering the dealer overheads like ground rent/leases can't be as much in Stoke as they are here.
However, i think the reason they're quoting 72,000 mile service is because that's the service at which officially the cambelt should be changed in terms of mileage. Obviously it also has a time limit, which you've reached before you got to 72,000 miles.
Might be worth asking them to quote for just a cambelt & waterpump change and then ask to add a Bronze, Silver or Gold service
http://www.fiat.co.uk/Content/?id=2182 if your car's over 3 years old which i'm guessing it must be. Might be abit cheaper.
Failing that, you have dealers within closer proximity than i do here, so i'd get a quote from someone like BS Marsons as a comparison. Unfortunately the other dealer, Bostons of Milford closed down & is now another branch of Platts i believe, so they'd probably price the same as Platts in Stoke, though don't quote me on it!