A new light?

[Mike1158]

One light sourcem a HDRI painted on to a surface and placed below the domw in the roof, above the central cooking/laundry area.

No artificial lights or windows, no lighting tricks and no post pro.

[Mike1158]

Next view has four LED light units above the kitchen units at NEWS, aproxiamtely.

[ntxdave]

Have you considered making the under surface of the artistic an emitter?

[Mike1158]

I have tried this numerous times but frankly, the HDRI thing works better.

[Fletch]

Here's the scene lit by HDRi spherical sky, and I have removed the glass dome (that was only complicating lighting calculations).
I deleted the emitter surface which presumably had the HDR loaded into the material. It was facing up - out of the space, so I'm not sure how the light was coming into the space, honestly.

I created a single rectangular surface and intersected it with the opening on top, and apply the Emitter>Sky portal>invisible template. ( be sure for portals or emitters that the face normal is pointed into the space)

Darkened all white and light colored materials, Increased exposure to 10.

[Mike1158]

Interesting stuff. The emmiter was painted both sides.

[Fletch]

yes, it was, but emitters are not two-sided.
I tested rendering your scene with nothing loaded into the sky slot and there's close to no light in the scene. So the light was only coming from the HDRi sky, it's going to render faster with less noise if you use portal in that lighting situation with unbiased rendering on Easy 09 or 10

[Mike1158]

Ta mate, I'll get there.