HIGGINS/Plymouth

I am trying to find a marriage certificate for my grandparents Thomas Higgins (born 1921, Ayr, Scotland) and Doris Higgins, née Cruickshank (born in Gateshead). I think they were married in April 1944 in Plymouth. My grandfather was in the military so the year could be slightly out. I’ve checked most Catholic churches around Plymouth and the GRO but there seems to be no trace of this marriage. They emigrated to South Africa shortly after getting married and lived in Johannesburg.