I replaced spark plugs at about the 100k mark. I have noticed my fuel economy has dropped over the past 20-40k but was assuming its dirty injectors / old truck problems. Was going to buy a magic in a can snake oil fuel system cleaner on the oft chance it will work. Perhaps I'll try the thermostat housing as well but $120 is a lot to risk without an idea for success.
I agree about the $120 and the new temperature sensor you get with all the plastic might not be the problem - but it might be. With a Land Rover, all fixing is a gamble - that is what makes them interesting; they are more a vehicle of choice for the gambler so to speak. All sensors deteriorate over time and for the most part, the engine computers compensate by defaulting to some predetermined nominal value once the sensor gets too out of wack. That means that in all older vehicles, all the sensors read less than ideal and hence the engine computer at some point in time is sending out default values rather than measured values.
The engine computer takes signals off both sensors, melds them together and then sends a signal to the transmission computer for it to do whatever it does. The combined signal also goes to the dash temperature gauge but the gauge so damped as to really only show cold, med and hot - of not much use really.
What I think is a gamble however is old plastic. How long a thermostat lasts is also a gamble and the new one might be a "cold" one or I suppose a "hot" one relative to what you now have. I have probably replaced that housing about four times now - the last one was a cold thermostat and the current is what I would call hot, relative to previous. Up here, hot is good as that means the heater puts out more heat in the winter and summer is kind of a blink in time anyway.
For me when the ZF started hunting, the solution was replacing the factory installed temperature sensor by the oil filter as the thermostat located ones are always relatively new.
I got lucky - my gamble paid off.
For me, the bigger gamble would have been not changing out the thermostat housing regularily.