Pete wrote:
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.

such drill rigs are available. I've seen them being used.

google search: horizontal drilling rig 200mm http://snipurl.com/mis4

do a bit of ringing round, you will find one. alternatively, give this lot
a ring / search their site http://www.drillandsaw.org.uk/ someone in
your area will be able to quote you for doing the job.

if the outer wall /is/ concrete your best option /is/ diamond drilling.