I am hoping the really good tiler we used a few years ago, who lives just round the corner, is still about (I wouldn't blame him if he's returned to his native Hungary!) - I'm inclined to ask him to look at the tile and the fireplace and suggest how he thinks it might work best. It's a bit awkard as they are big tiles (20cm), and either side of the grate is 18cm, and to get it even you'd need, I think 4cm of tile, then 14cm of tile on each side and I don't know if that might look a bit shonky, so I'd value his opinion on it.
Sugar Kane - the house was built circa 1911, I guess it's possible someone bought it an older surround for the front room at some point. I can't remember what our neighbours have in their front roonms.