What floor is this fireplace on?
The hollow behind the missing brick appears to be soot free - am I right?
Presuming that the RH side of the c/b is also single brick then any widening would probably need a couple of Acrow Strong Boys to provide support while a lintel is installed.
The hollow "legs" must be solidly bricked up in line with the back party wall - FWIW, I've never seen this before.
Read my (or others) recent past posts on opening up and installing appliances.
Essentially you will have to work to appliance Mfr's dimensions & have HETAS involvement.
note: remove the carpet and tack bar.