Manually control note spacing?

[Robert Hickman]

Hi,

I'm trying to make an example showing mordent symbols with text
labels. I want the note positions spaced so that the text is all on
one line, with notes placed as needed to fit around it.

The easiest way I can think of doing that would be to put manually
sized 'gap fillers' between the notes which is something I often do in
LaTeX, but I can't find any way of doing that in lilypond - it only
seems possible to change spacing globally?

\version "2.18.2"

\header {tagline = ""}
{
    \numericTimeSignature
    \time 4/4
    a'4\mordent^"Mordent"
    a'4\upmordent^"Upper Mordent"
   a'4\downmordent^"Lower Mordent"
}