If it's just one hole, something like this would do it:


http://tinyurl.com/bpzyw


If you want several holes done at once, use one of these:


http://tinyurl.com/acghx


Well, I am not quite sure that these recommendations would produce the
neat hole I was hoping for...

Anyway, thanks for all the advice.

I have created the hole today, and all went well. In the end I used a
technique that was a combination of the advice I received. I hired a
core drill (with various diameter bits) and bored a series of
interlinked 2" holes around the perimeter of a 7.25" circle on the
inner wall. I used my own SDS chisel to remove the remaining masonry
around the periphery to leave the desired 7.25" hole.

From inside the house I then drilled an 8 mm hole through the outer
wall from the centre of the 7.25" hole, and scribed a 7.25" circle on
the face of the outer wall using a measured piece of string tied to my
8 mm drill bit.

The inner wall had been a lot tougher than expected - not just a normal
breeze block. When drilling my 8 mm hole I had found that the outer
wall was even harder, and it would have taken a long time to bore seven
2" holes throuth it. I therefore chose to bore a single 6" hole (the
largest diameter borer in the kit) in the centre of the 7.25" scribed
circle on the outer wall, and then used the SDS chisel to remove excess
material.

Things went very well. The holes in both the inner and outer wall
blocks were pretty neat. Not as neat as a bored hole, but not far off.
The gaps left around the pipe will be easily filled.

Again, thanks to everyone who contributed; your advice enabled me to
assemble the right equipment in advance and use a technique that
produced a good result. It didn't take long to do, either.

Cheers.