I'm not saying that the td5 is not a good engine, infact it's a very good engine proberbly the best in any 4x4 but what happens if your in say Morocco in a td5 that is 4years old and with 120k plus on the clock and it breaks down. Your local mechanic is unlikely to have a lap top and the software to find out what is wronge.
A defender is not just a vehicle but a statement of your life style. The people know that if you buy a new Defender that it will, if they want it to last them for at least twenty years, and more if they look after it. It's rugged image is what sells defenders, lose that and you lose sales! If you are unlucky enough to break down far away from any land rover delership you need to know that any local mechanic can get you going again if not yourself.
Hubber, you said that yours is "a joy to drive", well yes, it's a land rover thing. Every time that I get behind the wheel of mine it makes me smile, and 99% of defender owners would agree I am sure. It's not many vehicles that can clame that is there. I have driven or owned Land Rovers all my adult life and would not change any one of them for anything else.
One more thing, the British army do not have td5's but still go for the tdi's. Worth thinking about that isn't it.