I thought that impellors were all plastic these days ?
A mute point really as the timing belt is likely due anyway - an easy way to find out for future reference perhaps ( hint ).
I too would suggest the thermostat as the first port of call.
Happy new year to all .......
Is this a non-AC model?
If so, then the thermostat is the most likely culprit.
On Non-AC models, the heater is just a small radiator on a parallell loop to the normal radiator.
(Hose connects to the thermostat housing or near it, but on the other side of the thermostat than the top radiator hose)
Water is always circulating through this secondary radiator, but because of small size and little 'heat loss' it doesn't impact engine temperature all that much.
Now, if the water is always circulating through the big radiator up front... even when the temperature drops below the point when the thermostat should close... then the temperature keeps dropping like nothing else...