Petra, when you looked at www.iomfhs.im did you look at Brian Lawson's index as well as the ex-missing Braddan volume listed separately?
www.iomfhs.im/lawsons/LawsonBMD/burials/cx_cowell.html?Submit=Cowell

I would also try Cowle(y): www.iomfhs.im/lawsons/LawsonBMD/burials/cowley_5.html?Submit=Cowle%28y%29%2C+Tamar+-+Winston

Sue