Thank you both Valda and Linda for your prompt replies, it has shown me a new line to pursue. I presume that Susanna not being present at 1861 Census as Wedding cert shows.
Valda's querying enumerator error could be because of William Ritchie's Scottish accent, because place of Birth shown as Scotland in 1851 and Musselburgh being entered as Merrselbury on 1861 Census. I queried this with the Scottish Office.
Regards
Eryl