How can I fix my heaing on the car ?

Expansion tank needed filling up twice today after driving only 26 miles. Yesterday I noticed the heating was cold. This moring I topped the tank up. I noticed no leaking pipes or joints, never saw any water on the floor. Can,t find anything useful in the maintenance manual or diagrams. What can logically be the cause ??I have a rover 45 about 7 yrs old. Any ideas please.