Yes, it is Marylebone.
You can use Brett Langston's 'Civil Registration Districts in England and Wales' website, which is arranged by Registration District, but also includes a Place-name Index.
www.ukbmd.org.uk/genuki/reg/Alternatively, sometimes you can use The National Archives' Catalogue;
www.nationalarchives.gov.uk/catalogue/search.aspby putting in the place name and restricting your search to the record series for the nearest census year - 1841 and 1851 HO107; 1861 RG9; 1871 RG10; 1881 RG11; 1891 RG12; 1901 RG13; 1911 RG14 (the census isn't released yet, but the catalogue descriptions are in place).
It doesn't work in every case, if the place is too small to appear in the Catalogue description, and for a name like St John's Wood you might have to try a couple of variants, or use *. But most of the time it works quite well, and gives you the sub-district as well as other parishes nearby. The census was administered by Registrars up to and including 1911, so the districts are always the same as for Civil Registration.
Good luck with your Smiths. I have Smiths
and Browns, in both England and Scotland, and I have found most of them, so it can be done!
Mean_genie