Thanks so much. Really helpful to get that confirmed in black and white. I've dithered so much on this, but I couldn't live with asymmetry and so have left it to pretty much match the other fireplace in size. But it's been tempting...

Hi, we would recomend a high strengh mortar cleaner. Something like "Azpects Easy Cleaning Mortar Stain Remover". Unlike any other acid/alkaline cleaners, it does not require neutralising after application. Its just a case of brushing it on, leaving it for 10 minutes, and then rinsing it off. I hope this helps

I'd go with a normal mirror, anything else will probably look weird, if you don't want to watch yourself eat, sit with your back to the mirror or make sure that when you are seated, the bottom of the mirror is high enough so that you can't see in it. After all, it would also be strange to have the bottom of the mirror the same height as the dining room table, as it would surely be too low down.

I'd paint or limewash all the brick, install some chunky wooden floating shelves in the top part and add some ventilated cupboard doors for the bottom.