Thanks everyone, great advice. Barrett's chart is very useful - cheers for sharing it.

I'm not sure I want to reduce down too far to the 40" range as I'm not a fast spinner.

I want to keep the double crankset so looks like its a straight choice between 12/23 and 13/26 cassette. Shame Campagnolo don't offer an 8 speed cassette in 12/25. I guess I could buy a 13/26 and custom build a cassette using my present 12/21? Or will I encounter shifting problems mixing a slightly worn cassette with new?