Thank you all so much for the time and effort you have put into helping me. The family were all strong Church of Scotland people, so Mary Ann marrying again, only 5 years after her marriage with John Duan in a Scottish church, to someone in a Catholic church in England, is surprising, especially as this man is a John too and with a surname starting and ending with identical letters D and N, but there are too many coincidences with names, dates etc there to discount the idea.
I will reread all your replies and look up the suggestions you have made for further research. Thank you again, very much. Best wishes, Islay