How old is your disco. We find that ours come on at start up too and switch off after warm up. During the summer they pretty much run all the time, and you're right their as noisy as all buggery.
If your gauge is not moving from normal operating temp then your fans are doing the right job and keeping the engine at the right temp. Its when your temp gauge goes up (or down to cold) that you know their buggered. We've been contemplating putting some electric thermo's on with the idea that they may be quieter and we won't get the huge drop in power when they come on but we're yet to actually do something.
Edit* just re-read. If their coming on on the highway (ours do this too and we take it as normal) you may have crap blocking the flow through the radiator. We've got 2 spotties that may be contriputing but if you've got a whinch, spotties etc it may be affecting flow.
Edit x2* looked at your sig an see its a 95 model. Our is 97 and all ok.
I think its just another landrover thing. You're just too fussy
