smaller distance between first Staff and TimeSig

[Stefan Thomas]

Dear Lilypond-users,
I have the problem, in the below quoted example, that I can't reduce
the distance
between the TimeSig and the first Staff of the Score.
How can I do it? I tried it with
\override VerticalAxisGroup #'keep-fixed-while-stretching = ##t
but without success.
Here is the snippet:
%%%%%begin%%%%%%%%%%%%%

\version "2.12.1"
\paper {

  ragged-last-bottom = ##f
  %{%das letzte System kann unter Umständen einen Seitenrand nach unten belassen
  top-margin = 20 \mm
  right-margin = 8 \mm
  left-margin = 8 \mm
  bottom-margin = 12 \mm
  horizontal-shift = 2 \mm
  between-system-space = 7 \mm %sorgt fuer ordentliche abstaende,
zwischen den einzelnen Notenzeilen
  between-system-padding = 14 \mm %der Standardwert ist bei 4mm! Der
Abstand zwischen den Systemen
  page-limit-inter-system-space = ##t
  % das heißt: der Maximal Abstand zwischen Systemen wird begrenzt
  page-limit-inter-system-space-factor = 1.3
  %er kann bis zu 30 Prozent mehr betragen
  %das heißt:
  #(set-paper-size "a4" )
  %}    system-separator-markup = \slashSeparator

}

 \layout{
  \context {
    \type "Engraver_group"
       \consists "Time_signature_engraver"
    \consists "Text_spanner_engraver"
    \consists "Text_engraver"
    \consists "Dynamic_engraver"
    \consists "Axis_group_engraver"
    \name "TimeSig"
       \override TimeSignature #'font-size = #3
    \override TimeSignature #'break-align-symbol = #'staff-bar

    \override TimeSignature #'X-:set =
#ly:self-alignment-interface::x-aligned-on-self
    \override TimeSignature #'self-alignment-X = #CENTER

  }
  \context {
    \Score \accepts TimeSig
  }
  \context { \Score
    \override VerticalAlignment #'max-stretch = #1000000
  }
  %evtl ist es sinnvoll, den Time-Signature-Engraver aus der Partitur
zu entfernen!
  \context { \Staff
   \remove "Time_signature_engraver"
  }
}
global = { \time 3/4 s2. \time 4/4 s1 \break \time 5/4 s4*5 \time 4/4 s1 }
melodie = \relative { c4 c c c c c c c c c c c c c c c }
instrument = \new Staff { \melodie }
tempotakt = \new TimeSig \with {
        \override VerticalAxisGroup #'keep-fixed-while-stretching = ##t }
      { \override Score.TimeSignature #'style = #'( ) %heißt: die
Viervierteltakte werden nicht altmodisch angezeigt
                %\override Score.BarNumber #'break-visibility = ##(#t
#t #t) %jede Taktzahl wird gezeigt, spaeter wieder loeschen!!
                \override TimeSig.VerticalAxisGroup #'minimum-Y-extent
= #'(-0 . 1)
                \global }
\score { <<
\tempotakt
\instrument >> }