I believe this is a common problem which I've experienced for over six year until the screen went totally blank. In the past after a flat battery I generally had to pull the fuses/relays in the rear right panel to reset the Nav, however this is different issue.
The Nav supply power wasn't constant and was influenced by moving it's cables indicating a short. Eventually realised it was the Blue connection plug wasn't fully making, removed it's outer Blue cover and attached plug without it, Nav operation now fully restored.
I filed the facia of the Blue plug so to limit it's standoff distance and reassembled, so far all sound. I trust this may help others with similar issues.

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