When should I change Cambelt on VW 140 bhp diesel engine?

If you have had the car from new and know its service history then 40,000 miles is the norm. But it may also depend on the engine type fitted, some don't need changing until you hit 70,000 miles. If you don't know the service history and the car has quite a high mileage, CHANGE IT! Far better to pay £150-£250 on a belt change than have to pay for a new top end if it breaks. This is classed as an "unsafe" engine , meaning if the belt breaks the pistons WILL hit the valves and cause a great deal of damage.