I had the same story with my 2001 TD5 Defender, the viscous fan was running all the time, you could hear the noise and feel quite a strong blast of air all the time the engine was running.
Fitted a new one which you can hear running when it first starts but if I drive down the street then back the fan is slipping and the noise is very different, even with the bonnet down.
Result is the engine now warms up much sooner probably less than two miles ( winter) and the heater gives out some heat.
Heat is a relative term for a defender, but certainly better than before.