Installing lintel in thick stone wall

I've done this type of job on 20inch stone walls a few times, and there's a lot more work involved than doing the same in a cavity wall.
Is the exterior face stone or rendered?

 

If I was doing the job, I'd probably use 2 needles and 4 acrows if it's a random rubble wall. Have to knock out a bit wider each side, than rebuild the jambs with brickwork. Then put the lintels on and pin up to the wall. Come back another day and remove the needles and fill in the holes. Coup!e of days work for me and a labourer to start, and half a day to come back.

 

At the other end of the scale you probably have the chancer who thinks it's a quick disc cut, no acrows and bish bosh.

Stuart's is the right way but obviously costs a lot more. I'd even prefer to see a couple of days between bricking up the reveals and packing the lintel but that is gilding the lilly a bit.

 

Leaving a few days to allow for a bit of settlement and shrinkage is technically the correct method, and would be used if an engineer was overseeing the job.

 

I'm curious about the stone wall being internal, only seen this on old farm buildings, I can maybe give a couple of examples but the internals were brick and the sandstone ones were exterior, my external ones took a day to dig out ready for the lintel(6 feet wide) a second day to put the lintel in and cut the doorway, a third day to point and clean up, the clean up was a bit like cleaning up after a cruise missile.
The interior ones took a day each to put the lintel in and knock the door out, then a second half day to cement and tidy but it was -5 and the guys barely took a break from start to finish, and the brick made it a way simpler job than the sandstone.