Tempo marking in score

[Michiel Lange]

To show the tempo in a piece I have added the \tempo command in a 
ChoirStaff like this:

   <<
   \context ChoirStaff<<
       \context Staff = women <<
       \key ges \major \clef violin
       \set Staff.midiInstrument="choir aahs"
       \context Voice= sop
       {
           \tempo 4=72
           \relative c''
           {
               \voiceOne << \sopm >>
           }
       }
       \context Voice=alt
       {
           \relative c''
           {
               \voiceTwo << \altm >>
           }
       }
  >>


The result is not really what I would like as you can see in 
http://minas-2.demon.nl/~michiel/blessyou.preview.pdf

I would think (but that's just me I guess) that a \tempo should be in 
the header of the piece (?) this did not seem to work though... (the 
header really disappeared completely)

This header is not valid for example:
\header
{
   texidoc = "The Lord bless you and keep you"
   title = "The Lord bless you and keep you."
   subtitle = "(De Heer zal u behoeden)"
   poet = "Ned tekst: Frits Ham"
   composer= "John Rutter"
   \tempo 4=72
}

How can I prevent the rest and the tempo mark to collapse?

Michiel