Ford Fiesta 1.4 Duratorq TDCi (68PS) DEC 2009 - Ford Fiesta Oil Pump Seal v Cambelt replacement -
If these engines are similar to most modern Ford engines there is a very good chance that an oil leak from the area of the oil pump will have contaminated the cam belt. Not to replace the cam belt is asking for trouble since oil will affect the rubber. A new belt costs pence in realtion to the damage that would be caused if it broke or slipped.
The garage are probably being very sensible in their insistance on changing the belt.