I'd be temped to paint it white and either put a black cast iron insert in or line out the opening with brick (love the herringbone example shown below) and put a wood burner in. http://www.chesneys.co.uk/products/accessories/interior-panels/ches-brickherin

I personally would not as it might 'peel', it might not, but better safe than sorry. Let the paint thoroughly dry and if it's the same base ie Acrylic, which is most likely these days, then sand and re-paint. If you do not wish to sand, or they are different bases, then wait for the paint to thoroughly dry and then use Zinsser 1-2-3 primer or even Zinsser Peel Stop to go over the gloss and then paint with the Matt.

I’m no expert but if you want a nice clean finish it’s best to get a plasterer to skim the walls. You could try some filler like Polyfiller in the dips, let it dry hard then gently sand smooth.

Aw shucks guys! Thanks! I’m definitely aiming for a tiny bit more stylish. My partner brought that heinous green drainer from his house and was everso pleased with himself for solving a problem I wasn’t aware I had! And the el cheapo Wilko kettle is a placeholder til I get a boiling water tap, so I haven’t upgraded it for three years. Plus he’s got a brewing corner in the kitchen, and a 3D printing bench in the utility. I’m just glad to have his wallpapering skills! Though we had a difficult conversation when he replaced my Henry who fitted under the stairs for his Dyson which doesn’t :-)