Think I've answered my own question, just found this..
2. The Freelander Drive Train
a) Viscous Coupling Unit (VCU)
i) In the Service History
The viscous coupling unit (VCU) is a sealed unit on the prop shaft that supports the four wheel drive function of a Freelander. All Freelander 1 models have a viscous coupling unit (unless the prop shaft has been removed, rendering the Freelander a two wheel drive vehicle).
The viscous coupling unit is a sealed unit which contains a viscous fluid. As the viscous fluid becomes thicker it over time the viscous coupling will become stiffer to rotate and this will put a strain on the whole drive train of the Freelander. Hence the viscous coupling unit is an item which should be replaced regularly, at about 70,000 miles. If there is no record of having the viscous coupling replaced and the Freelander you are buying has done 70,000 miles or more then you should be considering replacing this immediately.
HOW MUCH WILL IT COST?
A reconditioned viscous coupling unit, which has had its viscous fluid replaced, is £270 (add an extra £65 if you replace the bearings at the same time, which is recommended), and fitting is £105.
ii) On the Freelander
Since the viscous coupling unit is a sealed unit it is difficult to know if the viscous fluid has become too thick it is not like the oil where you can pull out the dipstick and see how it looks. If when you drive the Freelander it feels as if it is holding back or as if the brakes are binding, then this could be the viscous coupling showing excessive signs of wear. It will be especially noticeable when reversing on full lock.
Once the viscous coupling becomes stiff there is a lot of strain put on the whole drive train. If the Freelander is still driven as normal it will eventually damage the IRD unit (transfer box) and the rear differential then things get really expensive. Generally if you can hear a clunking noise from the front of the vehicle as you go around corners then there is a good chance that there is damage to the IRD.