With a lot of googling I've answered my own question, and will post the answer in case anyone else stumbles across this. The Galaxy CO2 emissions were reduced with revisions introduced 2012. So older 163ps (and 140ps) manuals are in Band G at £175, newer ones are in Band F at £140. Interestingly all are Euro 5 compliant and there is no change in performance, so possibly a lightly revised engine map. This can be seen by putting different aged cars into here and then checked against an old brochure.