It is the heater control solenoid, situated under the scuttle trims at the rear of the engine. They are electrically operated solenoids, that open and close to allow the water to the heater. They can be found on Ebay for about £15.00 and can be changed in about 1 hour by the average DIY mechanic.
Don't let the engineer take you for a ride, if he had to go away and think about it, he is not much of an engineer.
Graham
November 2009