More vertical space between staves

[seppo silaste]

With the generous help of Carl D. Sorensen, Risto Vääräniemi and Kieran 
MacMillan I've created the following empty sheet with empty fret diagrams:

\version "2.11.64"

\header {
  tagline = ""
}

\storePredefinedDiagram \chordmode{c''}
                       #guitar-tuning
                       #"o;o;o;o;o;o;"

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

\score {
<<
 \new FretBoards {
   \override FretBoard #'fret-diagram-details #'open-string = #" "
   \override FretBoard #'fret-diagram-details #'fret-count = #6
    \chordmode {
      \repeat unfold 12 { c''2 c''2 }
    }
 } 
 \context Staff {
   \repeat unfold 3 {s1 s1 s1 s1 \break}
   \bar "|."
 }
>>
}

Only problem is that there is too little vertical space between the 
staves/lines. How do I increase that (and add more space at the top)? As 
a purely academical question I was wondering why lilypond will give an 
error warning even though the document is fine? This is the lilypond output:

Interpreting music... [8]
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `blues_2.ps'...
Converting to `./blues_2.pdf'...
error: failed files: "blues_2.ly"

regards: Seppo