Diagnostic: Mood Tag
Halliday & Matthiessen (2004: 241):
However, note that clauses such as on the north wall … hangs a Union Jack … are not ‘attributive’ but ‘existential’. The thematically unmarked form of these clauses is that beginning with existential there: there is (hangs) a Union Jack on the north wall. The prepositional phrase then appears initially as a marked Theme; in that case the existential feature may be left implicit, although the there may still be present and will appear in any case in the mood tag: on the north wall (there) is a Union Jack, isn’t there?. In contrast, in a ‘circumstantial attributive’ clause, the Subject/Carrier is picked up in the mood tag: the sounds and smells of the ocean hang in the air — don’t they? …
