Dinny: I really love the Think Tank Retrospective 5 and 10. Much better quality and comfort than my old Crumpler. If you wanted some thing really compact, you could get away with the 5 but it won't hold much more than the 10. The 10 will give you lots of room for camera, lenses, flash etc. I found that the 20 was much too large for me. Think Tank products are really great. If you want to also carry an ipad or 11" Air, you might want to wait for the 7.
http://www.thinktankphoto.com/categories/shoulder-bags/retrospective.aspx