Enlarging space between systems

[Jogchum Reitsma]

Hi,

Running Lilypond 2.19.82 on Opensuse Tumbleweed, I want to enlarge the 
space between systems of choir staffs.

Looking in the docs, and googling, I tried

\override VerticalAxisGroup.staffgroup-staff-spacing = #'(50(50))

and

\override StaffGrouper.staffgroup-staff-spacing.basic-distance = #15

in the layout {} section.

But whatever values I place in these commands, nothing changes in the 
spacing between the systems.

Anyone to tell me what I do wrong here?

regards, Jogchum