Does Aunt Maud think that repairs just need a bit of plaster filler or ripping off?
I'm always reluctant to go backwards and re-do simple things which should have be done right the first time.
In this case, I would contact whoever built it and ask them to sort it out. If they were experienced in fires and surrounds, they would know that putting a joint like that on a piece of steel, just there, is the work of a numbskull.
I think it's highly likely that there will always be a crack there because the joint is in a place which is subject to a wide range of temperature and the piece of steel will expand and contract every time the fire's lit.
Personally, and as I live in a crumbly pile anyway, I don't mind cracks in plaster as they are my cracks in my plaster and get filled when I lime wash the walls and ceiling. I don't mind a bit of patina either, but I would be cheesed off with a crack just there, more so if I had paid for it not to be there.