SCAREY-B.HERE-ON-THE.NET wrote

What's the construction and thickness of the wall?

outer layer standard brick with a 70mm cavity and iner layer standard brick

Certain it's 70mm? The internal brick tells me it's a fairly old house and
cavities were virtually always 50mm in those days, although there could have
been some build inaccuracy or some bulging of the outside skin. Anyway, either
way the easiest way to do this is with a Catnic CN7A lintel (for 50mm cavity) or
CN3A (for 70mm). If you can slide the lintel up inside the cavity you could use
a CG50/100 or CG70/100 which saves a bit of work.

A Catnic is the best for this job as it does not need a separate cavity DPC
above. It's 150mm deep and needs to be min 300mm longer than the width of the
opening to give 150mm bearing each end. They come in 300mm increments in
length.

You're going to need an access tower and hire 1 or 2 Jackall wall supports from
HSS to support the wall while you cut out the bricks to insert the lintel.
These will save having to support the brickwork all the way down to ground
level.

see www.catnic.com and look for Products Lintels Cavity Wall Combined Box
Lintels or Open Back Lintels

Peter