vertical spacing - distance staff and group

[Hans van Eck]

Spacing question: does anyone know how you can change the vertical 
spacing between the Altsax and the Piano? (lift it a bit) I tried all 
kind of edits copied from the manual but I could only change the spacing 
inside the piano-group.

How do you do that?
Thanks,
Hans van Eck


code:

\paper {
  #(set-paper-size "a4")
}

global = {
  \key c \major
  \time 4/4
}

altoSax = \relative c'' {
  \global
  \transposition es
  % Muziek volgt hier.
  a4 b c es
}

right = \relative c'' {
  \global
  % Muziek volgt hier.
  a b g f
}

left = \relative c' {
  \global
  % Muziek volgt hier.
  des bes ges f
}


altoSaxPart = \new Staff \with {
  instrumentName = "Altsax"
  midiInstrument = "alto sax"
} \altoSax

pianoPart = \new PianoStaff \with {
  instrumentName = "Piano"
} <<
  \new Staff = "right" \with {
    midiInstrument = "acoustic grand"
  } \right
  \new Staff = "left" \with {
    midiInstrument = "acoustic grand"
  } { \clef bass \left }


\score {
  <<
    \altoSaxPart
    \pianoPart
  >>
  \layout { }
  \midi {
    \context {
      \Score
      tempoWholesPerMinute = #(ly:make-moment 100 4)

    }
  }
}

\layout {
  \context {
    \Score
   \override StaffGrouper.staffGrouper-staff-spacing.padding = #0
   \override StaffGrouper.staff-staff-spacing.basic-distance = #18
        % no effect
  }
}