The brightness of the dash has become 'aware' and has a mind of its own. But only sometimes so it obviously has to sleep.

When it wakes the display goes to full brightness and the brightness control does nothing. Happens day and night. Anyone else have this? 

Yup - happens on mine too and mainly at night when driving through streets brightly lit up.

Jas Current RRS: MY18 SVR
Previous RRS: MY15 RRS 5.0L Autobiography

Is it clever and can sense light , so it will get brighter in lighter conditions and dim when it's dark so as not to be to bright .

Fletcher, that's right.

I think the issue is that it is far too sensitive to changes in light conditions.

Jas Current RRS: MY18 SVR
Previous RRS: MY15 RRS 5.0L Autobiography

and then of course if the infotainment screen is set to stealth everything is dimmed and non adjustable till you put the screen back on.

Nice idea - would rather someone hadn't thought of that though....

yep, mine is all over the place too, sometimes so dim in the day you have to guess the speed, but it's still not annoying as the heated steering wheel that turns its self on from time to time (and no, I'm not hitting the button by accident)