Hello John,
Thank you for your reply.
I have been in touch with another member recntly, who gave me feedback on this marriage. As the Mercer people that I am investigating probably lived and stayed in London,it seems now somewhat unlikey that this was their marriage. However, I can not find any entires for Charles and Harriet's marriage on the research indexes. I am assuming the period would be 1860's - as I see the ages on the 1871 census, with their son Henry, born about 1867/68. So a bit more delving is needed! But many thanks for getting back to me.
Regards
PatB