Fentons supplied and fitted ours. I had to considerably modify the opening, increasing its height by 18" to allow the fire to be 14" off the floor, and widening by about 7" each side. They wont undertake that work, they just want to fit it into the space, install the flue and connect the gas. Maybe they do more of that end of it now, but they didn't when we got it installed.
Yep the fire is just installed flush to the plaster on the fireplace, no mantle or hearth.
I cant stand the look of the 'Alcatraz' wire fire guards, so the central heating has to suffice for us for now.
Best of luck,
John