Thanks everyone for the really varied and interesting comments.... I'll keep going and let you know how I get on...
One question for Raisinberry777.... when I mentioned 11/23, you said 11/27 or 12/28 as being a better climbing ratio.... I wasn't aware that you could do 11/27. Is the idea behind 11/27 the fact that you can keep a really good spin going up the hill, while not losing top speed when in big cog for going downhill. IN which case, why do the manufacturers make an 11/23 at all?