And another thing..... Looks like I may have answered my own question about just how much additional locking the ELD button does when pressed. I found a big gravel/mud bit of the station carpark yesterday and put the steering on full lock and drove dead slowly without any buttons pressed. Silence prevailed in terms of 4WD type noises and car turned easily.
Then I hit the ELD button and I could feel a gentle 'stiffening' of the steering and progress, nothing much, but enough to detect a change. In addition the sound changed and I could hear a very slight pulsing from the read diff - a low, subtle noise - nothing to write home about, but definitely there and obvious.
So I now remain convinced that despite what we might think, the ELD button doesn't actually lock the thing solid, but as has been said by HH and Goudrons, merely primes the diff to be ready to react quickly, should it be needed. The stiffening and light noise are evidence of that.
Either way, I still need snow to make this a proper, proven experiment!