wozzy, we really need more information and, if possible a picture or two.
Can you be clearer about the type of fireplace, what function the rendering is to perform, what material the fireplace is built of? Will you be lighting open fires in it? Or is the rendering to be purely cosmetic?
Is the house listed? How old is it?
Despite what you might now be thinking, we don't need your inside leg measurement. But I do assure you that all the above questions are relevant to the kind of answers you might receive.