Check you are not losing coolant, first and foremost, and then check to see if you have a thermostat, as it sounds very much like mine.
I first noticed something was wrong when my heater started not being hot, experience told me to check water straight away, Gone !!
Then the question was "where's it gone", first thing then is is it leaking internally (head gasket) or is it leaking externally (one of a dozon things).
Start trying to trace it, and best case senario is you have no thermostat, which then raises the question "why".
I better point out that all my limited knowledge comes from my own experience and shed loads of landrover forums, so no doubt others on here will be much more informed than me
but please let us know what you find, good luck
Gary