Hi there.
I have enjoyed evesdropping your correspondance and think that in all probability, St Josephs of Elm Grove was the right church, for it is the only Catholic Church in the immediate area.
You may also like to know that from the 1880s the church had a school for Catholic children opposite it, and most of their parishoners sent their children there. That school was then relocated to Davey Drive in Brighton in 1957, and I used to teach there. Before their original registers were sent to the County Archives in Lewes several years ago, I wrote a pocket history of the original school which might also prove of interest. You should find a copy in St Josephs in Davey Drive, Brighton if you feel other family members also went there.
Roy G