It is possible that Clement and Elizabeth are in Wisbech in 1901 - at least, Ancestry seem to have made a bit of a pig's ear of Wisbech enumeration district 6.
Seaching for Longs born in Wisbech 1891-1900 (in case there were any children to find) I tried to look at an E R Long b 1900, who is indexed as a grandchild of 'C'.
The census image this links to is a single page of Wisbech St Peter's (no previous or next pages to go to). E R Long is not on this page, although oddly, it includes 18 Canal Street, where Elizabeth's bother John Bouch is recorded. Her brother George is on the same page.
I tried going through Wisbech district 6 page by page and couldn't find any Longs - but the pages are muddled, and the district description pages (which should be linked separately) are mixed in with them.