Firstly, thank to all who have taken the time and trouble to answer so far. (To Jim your answer kind of answers my question though some clarification may be required)
1) Permit me to explain how I arrived at the -2 stops = black and + 2 stops = white. Hopefully you can see how my thinking developed
2) Allow me to explain what I did to establish what my camera could apparently see. Both elements may be fundamentally flawed but they made sense to me at the time.
Tackling 1 first.
a) My exposure meter apparently covers a range of -2 1/3 to + 2 1/3 stops. Why would nikon do this if the camera can see 10 or 14 stops?
Reading about the Zone system reveal provides the following:
b) For us digital photographers, we are only concerned with zones III through VII (zones 3 through 7). The darkest part of a scene would fall into zone III, while the brightest part of a scene would fall into zone VII. Anything darker than zone III would render as pure black with no detail (under-exposed), while anything brighter than zone VII would render as pure white with no detail (over-exposed). Source: http://photo.tutsplus.com/tutorials/shooting/understanding-using-ansel-adams-zone-system/
Both a and b above led me to the conclusion that there is a useful 4-5 stops between black and white hence why does a wider DR in a camera matter???
2) I tried a little experiment. I took a letter (B&W) and pinned it to a wall.I established a distance which would allow me to see the text nicely when the letter was photographed with a suitable exposure. My thinking (and this is where I may have gone wrong) was the ultimate test is if you can pull back black text back from a badly underexposed photograph and still recognise it as such then you can still resurrect shadow detail. Black on black ought to be the stiffest possible test. When there is no more information left in the mage then thats the low point of the DR.
Sure enough when I tried this I got a cliff like highlight rolloff beyond +2 2/3. The text very quickly become unrecoverable.
On the shadows the rolloff was rather more gentle but still, I got nowhere near -6 stops before detail simply wasn't there. At - 3 stops the text looked like braille. In fact if you hadn't seen the photographed letter you wouldn't know that the info brought back was text (therefore I see precious little point in trying to recover it). I never got to - 6 stops. I was seeing a white surface long before then.
This is why I asked the question and this is why I still wonder what the point is of cameras with larger dynamic ranges as apparently we are still ultimately constrained to the real world practical limits of -2 to +2 or thereabout of pure white and pure black.
3) Additionally (For Jim) I didn't get this bit of your explanation. Where did the SNR of 10 come from in your text below?
I believe DxO sets the signal-to-noise ratio at the low end to one. Most photographers wouldn't do that. An industry rule of thumb is that 100 electrons in the sensing element is the fewest that gives a "photographic" SNR.
With 100 electrons in an otherwise=perfect camera, you get an SNR of 10. So there goes a little over three stops off the DxO dynamic range. Now we're down to, for your camera, a bit under 9.
4) Finally I should say I'm not arguing with you chaps. I am simply trying to really nail this and understand. I want to be able to use my camera to its fullest extent and know how far I can push it. I also want to understand as much of the theory as I can.
If you choose to answer this then great but if not well thank you for your kindness so far.