I haven't got the marriage cert yet, just his birth cert so far.
I have quite a large DNA match to a descendant of theirs, and I was concentrating on Frederick's side first, as Florence was born in India, so that was going to be more of a problem or so I thought. But that's fallen out nicely in the last hour and I've found that Florence Penny is my relative after all and not Frederick.
The bonus is that as this DNA match and I have several shared matches at a similar level, so they're all likely to be my third cousins with the same CAs as well.