Thanks for all the suggestions.

£90 for one day's hire of a hole cutter is definitely more than I
would prefer, but I tend to agree with the comment that it would be
worth it to get a fast, easy and neat job. However, I checked with HSS
and they advised that the suggested cutter was for cutting holes in
floors and would not cut a horizontal hole through a wall.

One further complication has arisen - I have now been told that the
outer course of blocks might be concrete, and thus (presumably)
considerably harder to cut than breeze blocks.

Interesting suggestions from The3rd Earl Of Derby and Dave Baker. I
have also received a suggestion that cutting holes 2 or 3 inches in
diameter around the perimeter and grinding down the small pieces of
block left between the holes would be a faster way to go. This method
appeals, but I am not sure what might be the best means to cut away the
pieces of block if the outer blocks are indeed concrete. I guess I
could simply cut out the perimeter of a larger hole than I need so that
the pieces left wouldn't obstruct the pipe I am installing, but I would
prefer to make a neater hole than this if possible.

Does anyone have any opinion on this method, and a suggestion of how
best to cut away the pieces of concrete block left between the small
holes around the perimeter of the large hole?

Thanks.