Have you checked yourself for the usual signs like water in the oil etc. The v6 is not known for hgf like the 4-cylinders and many garages will just tar them with the same brush when they here you say "it's a Rover". Chances are it could be the water pump that is leaking, another faulty thermostat or you could be losing water through a faulty header tank/cap.
Regards
Boz
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