Thank you for all your comments and suggestions. Now I have a firmer idea of when the photograph was taken, I can start thinking about who it might be. I have two weddings - my great great grandmother Anne Leah married her first husband in 1858 - so if it is a wedding photograph that is a possibility - her marriage to my great great grandfather Thomas Grant was 1866 so probably too late - but Thomas was illegitimate - stated clearly on his wedding certificate to Anne and his father was listed simply as 'Elizabeth Grant mother' (she was also a witness), which suggests a very close relationship with his mother, so if it is indeed mother and son it could be them.
I will almost certainly never know who they are, but all your help and suggestions are much appreciated and I can now at least guess with more certainty.
Jackie
