I didn't think Defenders had ETC, I thought they had diff and Hi/Low transfer gear boxes?
Was it not just the other LR's that got rid of the diff lock and just had hi/low and an ETC button.
Basicly, don't use diff unless your stuck and I mean stuck. Pretty much you will just change the ratio on the transfer box when going offroad, or ripping a fence out the ground etc...
Diff should not be engaged on good surfaces, like tarmac as you will damage props and get wind up etc...
Find out what your Defender has, and I will gladly explain how to use it all.