The thing is with the modern diesel engines is they are so efficient even when at full temp on the gauge if lightly loaded/worked for a few miles they can actually start to cool down again, the gauge of course shows all is good but the engine can be 10 degrees lower than the stat opening temp.
Add in the fact the defender engien is so exposed even the air passing over the block is cooling it down, so yes a rad muff certainly helps, but and theres always a but they can get critically hot in the blink in the blink of an eye.
I have owned many land rovers and all took/take 4 to 5 miles at slow to medium speed town work to get the stat to fully open, obviously if you have faster roads you will get heat faster.