Thanks everyone - this is great help - everyone has been great
@Hallmark - I realise I was narrowing my focus on family searches in Ireland thanks - I will widen that out!
@Heywood - the witness names were not linked in any way I could tell but I will check the certificate again on that one!
@Fidlerslass - I need to check the certificate again as both families lived in Nelson Street in 1863 at the time of the Marriage so that John reference could be key. Also the Jacobus looks interesting - my link to George and Susanna is Jacobus (James) Davison born 1865 so there may be family names linking there....