Spacing - should be simple...

[AFK]

All, thanks for your hard work developing and supporting Lilypond.  What a 
project! 

This should be really simple.   The following snippet generates two pages of 
music with a page-header at the top of the second page:

\version "2.16.0"
\score {
        \new ChoirStaff  <<
                \new Staff { \repeat unfold 400 { c' }  }
                \new Staff { \repeat unfold 400 { c' }  }
        >> 
}
\paper {
    evenHeaderMarkup=\markup { \fromproperty #'page:page-number-string " - 
title (composer)" }
}

What I want Lilypond to do is to insert a little bit of white space beneath 
the header on page 2, directly above the first system on that page.  At the 
moment the music is too close to the header.  (This looks especially wrong 
if you insert a top-margin, or if the systems are widely-spaced.)

I've tried and tried, but just can't seem to get anything to do this.  I 
must have tried a hundred different things!   I promise I have read, re-read 
and tried to understand 
http://www.lilypond.org/doc/v2.16/Documentation/notation/spacing-issues  

I presume it's something to do with top-system-spacing (or system-system-
spacing, or score-system-spacing, or top-markup-spacing or ???????) in the 
\paper block, but everything I try has absolutely no effect.

Could some kind person short-cut me to what's presumably the single line of 
code that I need? Sorry if this is obvious, but I'm having no luck. 

Thank you very much!