Malcolm - it is easy enough.
You need to look at the images of the original GRO Index.
You can access this via Ancestry.co.uk but there are other ways.
Using Ancestry - login and click the 'Search' tab on the top menu bar.
On the right you will have a series of links and under the heading 'Historical Records' you will find 'England & Wales, BMD Index'.
Click on this to get to the links for Births, Marriages and Deaths.
Under Marriages there are two links labelled as 'transcribed'.
The third link is Marriage Indexes 1837-1983.
Click on this one and you will have a search box.
The only two compulsory fields are 'Surname' and 'Year'.
In the case of Joseph OATES you had supplied the following information:-
Named: Florence M DAVIES
Year: yyyy
Qtr: xxx-xxx
Spouse Surname: OATES
All you need to do is enter the Surname, Year and Qtr and submit the search.
From here you will be presented with an image of the Index from the GRO.
Then you simply scan down the page until you find the name you want and then compare the Volume and Page number with the information you had for Florence DAVIES.
That gives you the match.
Usually why you cannot find this in FreeBMD is because
1. The transcriber could not read the name
2. Said transcriber could not decipher the Vol/Page number
3. Said transcriber completely missied the whole entry
Years ago and long befoe most of this stuff was available in the form it is today I did a lot of searching manually.
I would find a Trewhella name and then scan the whole index for entries with the matching Volume and Page number.
Much easier nowadays!
If you do not have access via Ancestry then the other way to find these Indexes is to simply search UKBMD with Google.
From the list of options you will find at least one titled GRO Index or something similar and that should get you to copies of the original Index.
But be wary as I just found at least one site that gives you access - FOR A FEE!!!
There are free ones there though - so good luck.
CT