Well, the marriage certificate arrived today, which I think is pretty good going allowing for the holidays.
Unfortunately, it isn't exactly conclusive:
Frances Mary Broadwood is listed as a widow, which fits, but the certificate doesn't name her father - in the father's name and surname section is what looks like the word "George" but that has been crossed out and initialled "B" (I assume the registrar, Mr Barnes). There is nothing in the father's occupation.
I guess the next step is to get the birth certificate for her children - hopefully that'll give her maiden name, but what do people think - are Frances Mary Broadwood and Mary Hamilton Broadwood one and the same?
Interestingly the address of both parties is given as 47 Beaufort Gardens, Brompton so they were obviously living together quite openly.