F M P allows one to search the 1939 using both surnames to see if there is an Elsie, of any birth dates who moves from Clark(e) to Williams - just if you wanted to explore that option.
There might be a candidate in Pontypool but of a totally different birth date, and possibly born with Elsie as a second name.
Added: So that fits with the suggested marriage, but not with the birthdate at all.