OK chaps - suggestions please.

I need to cut a 12" diameter hole in a workshop wall construction of which
is a single skin of 9" x 18" x 4.5" 7 newton concrete blocks. Now I've all
sorts of drills, angle grinders, hammers and even demolition breakers, so
the problem isn't removing the material, the problem is doing it without the
wall collapsing !!!

Current plan is drilling a circle of (say) 1/2" holes with a masonry drill
and wiggling it from both sides to hopefully join them up without disturbing
the rest of the structure. (I'll probably select a spot where the hole is
fully bridged by an 18" block - position within a few inches doesn't matter.

Any other constructive suggestions very welcome.

(this is for a huge 3900 cuM/Hour extractor for a CNC plasma table down
draught exhaust)

Andrew