My disco never gets close to the halfway point on the temp guage - thinking my thermostat may be at fault based upon what i've read in other posts on here.

Is it a big job to replace? How long should it take to do ? Any potential pitfalls or tips i should know about before starting? Or any tutorials (did do a google search but found nothing)

One more thing .. where is it ?

Replacing a 300TDi CSW thermostat - LR4x4 - The Land Rover Forum

That is on a Defender but I can't think of any differences.

Mine always sits on 1/3 - I get worried if it gets any higher. It's not broke, don't bother trying to fix it.