Long shot
What do you know about William? There is a Maria Boorman death in 1890 aged 28. May not be her - but if it is - William may have remarried. Have you tried looking for just him on the 1901 census?
Thanks for your help with this - unfortunately William is proving a very frustrating mystery. The only concrete information about him comes from a note attached to the marriage record on the BMD, which reads:
St Dunstans Cranbrook
17 Oct 1885
William Boorman & Annie Santer
Fathers:
Robert Boorman - Charles Santer
So all I know is they got married in Cranbrook and his father's name was Robert Boorman - I don't even have his age for certain. There are four William Boormans on the GRO listed as having their births registered in Cranbrook between 1845 and 1865 (the most likely range) - two William Boormans born in 1851, and two William Henry Boormans (one born in 1855, and one born in 1856). In the 1861 census, there are only two William Boormans born in Cranbrook in the right range and the father of one is a William Boorman and the father of the other is a Thomas Boorman.
The most likely candidate for the one who married Annie Santer seems to be this guy who appears in the 1881 census, but I can't find a birth record for him to confirm who his parents were (from my search, there were no births of children called William Boorman in Cranbrook in 1861):
Name William Boorman
Age 20
Estimated Birth Year abt 1861
Relationship to Head Workman
Gender Male
Where born Cranbrook, Kent, England
Civil parish Guestling
County/Island Sussex
Country England
Street Address Guestling Thorne
Occupation Blacksmith Journeyman