Hole in wall help

[thornmproblem]

Advice needed please.

I wanted to get a professional to do this for me to save the hassle and make sure it’s done right, but had absolutely zero interest from any local builders so I’m considering doing it myself.
A gas fire was removed leaving a hole in the inner skin roughly 40x80cm. The existing wall is some form of concrete blocks, outer wall is brick, with roughly 50mm cavity that has some soft insulation in. Plasterboard is dot and dab.

My thoughts were:

• Cut back plasterboard around the opening
• Remove any half blocks
• Use Celotex or similar PIR board to fill the cavity – should stay in place and make bricking it up easier
• I think bricks rather than blocks will be easier to work with since they’re smaller
• Get some ready mixed mortar

Before I do any more planning, is any of that completely wrong? Is it OK to use bricks instead of blocks?

 

[jks7492]

They look like thermalite blocks. If you're planning to remove any half blocks anyway then it'll be quicker to just infill with new blocks which will course with the existing and be a lot quicker