One of the things that came out of the legal hoo-ha was this
Database of Reused Graves at West Norwood. No Hennings listed, so the disappearance of their gravestone must be for other reasons, but there are some Druces:-
Grave number: 168-78
Deceased:
George Druce buried in 1854
John Druce buried in 1840
Mary Druce buried in 1883
Susannah Hack buried in 1874
Thomas Druce buried in 1854
William Druce buried in 1847
William Druce buried in 1851
Grave number: 6532-62
Deceased:
Ann Druce buried in 1859
which I rather fear are among yours. The database doesn't say who's buried there now, and I think the plot number changes when a grave is resold, so a map will be needed to find the spots. I don't think you'll be allowed to put a headstone on a reused plot, but you could name them on a Hennings headstone with "buried in plot xxxx-yy" or "buried elsewhere in this cemetery".