Gap Between Flue And Brickwork/Plaster

Six months ago, I had a new Remeha 28c combi fitted. I was doing some decorating in the room where it was fitted, and pulled backet the large rubber grommet that fits around the flue where it passes through the wall.

I noticed that the half inch gap all around the flue where it passes through the plastered wall hasn't been filled.

I also checked under the outside grommet, and the gap between flue and brickwork isn't filled with cement either.

Is this OK? Do these grommets take the place of filling with cement or plaster?

Yes this is fine, look in your manual it will tell you whats required

I personally "prefer" to seal with sand cement/plaster and then cover with the seals, less chance of flue coming out accidentally for any reason at all.