Fireplace opening

Hello,

I'm planning on purchasing the below cast iron fireplace but need some advice in regards to getting it installed.

I've smashed through the plaster board in our living room to see what's behind (see below) and once the fire brick is removed there's plenty of depth for the deepest section of fireplace.

However the arch section needs to be recessed around 6cm and the current opening is too small. Around half a brick will need to be removed from either side and a row of bricks above - no deeper then 1 bricks worth however.

Do you think this will need a lintel or reckon we could get away without one? We're of course going to get someone in to do the job but if a lintel does need installing wondering whether the costs will climb considerably? Any idea on price would be appreciated too.

Thanks