Hi Anne,
Just joined and saw your query. Not sure if you since found this information but the marriage of John Mitchell and Muriel Jelbert took place in Gulval Church, 22 Oct 1936. If you go to the British Newspaper Archive you should find a notice of the marriage in the "Cornishman" newspaper, which may give you some more details. I have him recorded as John James Mitchell in my Jelbert family tree, so that may help you. Good luck, Ralph