If you don't know the church your Catholic ancestors attended, it is so hard to find them, for the reasons given in this thread.
I have found some of mine courtesy of info from a Rootschatter, kind Catholic authorities in London pointing me in the right direction and searching their registers for me, and luck!
I have found that my 3xggrandfather Thomas Martin, for his first marriage in 1793, married in the Catholic church (for his faith) and the following day in St Martin in the Fields (obviously for legal reasons!!!) So for those who know the names of your Catholic ancestors who married, you may find them on the IGI marrying in a C of E, and this may mean there was an (unofficial) Catholic marriage before or after.
Valda - could you clarify something for me, please? I had understood, perhaps erroneously, that Catholic marriages were legal after the Catholic Emancipation Act of 1829. If it was not till 1837, that could explain two of my Catholics marrying in St Pancras Old Church in early 1837. Do please let me know!!!
Thanks,
MarieC