There's a bit of an oddity for the new birth records for years 1920-1934. The mother's maiden surname isn't shown in the search results. However, if you enter the MMN in the search input, then the search results will include the MMN where it matches.
Example:
Desmond Ernest George Caves - born:1924, died: 1976, mother's maiden surname: Franklin
Search without the MMN, result:
1924 Q1 Desmond Ernest George Caves (mmn: ); Watford; 03A; 1342;
Search with MMN "Franklin", result:
1924 Q1 Desmond Ernest George Caves (mmn: Franklin); Watford; 03A; 1342;
FreeBMD and other sources that use the original birth indexes show the MMN as Franklin.
(Note: Desmond isn't in my tree, I was just looking for someone with a distinct name that I could test this with)
Another oddity is that when searching with a MMN, the results will also include records where the MMN doesn't match, and is blank in the results.