I bought a 1999 4/4 Zetec which had only done 16,000 miles and decided to have the cam belt changed by a local guy who does sports car engines.
He reckons biggest problem on low mileage, but 'old' cars is the plastic pulleys.
The plastic degrades over time and is prone to chipping off a piece of the flange making the belt jump off.
He has even seen chipped pulleys from stones thrown up from the road.

He fitted a Ford Kit which comes complete with belt and pulleys.

Howard