Opening up old fireplace

My 1930's house has the chimney breast removed from 1st floor upwards. The breast is present in the dining room with the fireplace opening bricked up by previous owners.
I want to open up to install a non-functional cast iron fireplace. I have chipped away a bit of the plaster at around 4ft and there appears to be a solid lintel so I have chipped the plaster around for the opening.
The problem is the bricks seem impossible to remove as they are bedded in with a very hard grey concrete-like substance rather than an easy to chisel out mortar.
What would be the best way to progress?
Brut force with lump hammer & chisel?
I am worried about hitting the things too hard in case I cause some damage.

Any advice will be appreciated,
Rob

hammer drill and a 10mm masonry bit
if the mortar is that hard that you can't drill it then drill lots of holes through one brick to weaken it
then hit it with the lump hammer, once you've knocked one line out the rest will come out much easier.

Thanks for the tip marshman.
I have been able to remove a couple of bricks (and a bit more plaster) but now I am in doubt whether or not a lintel is actually present.
What I originally thought to be the lintel was in fact hard grey plaster over the top of the original brickwork. Before I remove any more bricks I want to be certain there is enough support.
Where should the lintel be and how big are they generally?
See photos of WIP.
If lintel is required what can I use and where can I get from?
Also how is one fitted?

Thanks,
Rob

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1930's fireplace there will not be a lintel as such there might be a flat iron bar or curved one
but most likely a brick arch or bricks laid leaning outward each way with a key brick in the middle.
( don't know the term for this but you often see it over windows or doors)

maybe someone will jump in here and name this type of brickwork please

Hi. Had a look and cant seem to find any supporting lintel or arched brickwork. I'm guessing it was removed when they bricked it up.
What can I use for a lintel in this case (and where from) ??

prestressed concrete lintel 65 x 100 by 200mm longer than the opening, 100mm bearers each end
it don't matter if its a little longer but don't go less.

Builders Merchants.