hi
mines the same sits on the m all day long then drops down to the little box in the temp guage but my heater works fine but didnt as there was shit in the heater matrix
this could clear your problem up firstly drain the system of water
from in the engine bay locate the 2 pipes that alow the flow of water into heater matrix they are located around the back of the engine on the bulk head you will see one coming from the left and one to the right when found and system drained remove the 2 pipes carefully as its easy to brake the plastic tubes attached to heater matrix
then you will need a hose to flush the matrix through dont use a pressure washer with a bit of hose connected todo this as the pressure might be to much place a large container under the opposite end to catch what comes out which could just be sludge as the heater matrix is low down in the water system and are prone to getting soiled up with sludge
once the water is running clear reconnect the pipes back up and fill the system with water just for now no antifreeze yet untill you know the heater is working as it should ,
turn the heater control to full heat and put the fan on whilst the car warms up and watch the water level once the car starts and if it drops just top it up before the car gets to hot
if the heater works as it should which i think it will do drain system to add antifreeze job done
if it does not work and you are sure the thermostat is working correctly you could try covering half your radiator with kitchen foil which will allow it to get that bit hotter this is an old trick that use to work on the old tx1 taxes to get better heat in the winter but never ever had to do it to any rs as they run hotter than a n/a cars anyway
mark2