You might be looking for what they consider a bell fire. It's the refractory cast parts for a fire box. You can get custom cast iron from Pennsylvania Cast iron firebacks. But that will not meet code just using a piece of metal. www.copperfield.com is Americas largest fireplace parts distributor, they only sell B to B. but you can call them and ask if they know of a place that they ship bellfires or other parts to in the UK. You can possibly build it with firebrick but you have to use refractory mortar for it or it will break with your first hot fire.

You can build it using firebricks obtained from a builders merchants. This was the norm in the early fifties.

HI I am renovating my Victorian house uk and I want to use my bathroom fireplace, it has already been uncovered and the gap is 11" wide for the fireback, I have purchased a surround and cast iron front at 11" wide from the 18th century but cannot for all my efforts find either a clay or cast iron fire back 11" wide.

Any Ideas ;0)