Changing first-system indent

[David Sumbler]

I have finally got to the stage of tidying up the score I have been
working on.

The top-level .ly file is structured like this:

\book {
  \paper { *variables* }
  \bookpart { *titlepage* }
  \bookpart {
    \header { *variables* }
    \score {
      { *first-movement-music* }
      \header { piece = "I" }
      \layout { } }
    \score {
      { *second-movement-music* }
      \header { piece = "II" }
      \layout { } }
    % etc.
    }
  }
}

\book { *data-for-1st-violin-part* }

\book { *data-for-2nd-violin-part* }

% etc.


For the first movement of the score, I have Staff.instrumentName defined
for each instrument, and this requires an indent of 16mm.  However, in
subsequent movements I do not set Staff.instrumentName, so no indent is
required.

The variable 'indent' is defined in a \paper block, and I now see that
the lowest level context that a \paper block can appear in is \bookpart.

I could, of course, quite easily restructure the file so that each
movement is contained in a separate \bookpart, but this would mean that
each movement then starts on a new page.  This is not what I want: I
prefer the movements to follow one another with no page break to
emphasize the integrity of the whole work.

Is there a way round this problem?  In other words, is there some way I
can force the first system of the 2nd and subsequent movements to be
printed with no indent, without each movement necessarily appearing on a
new page?

David