How to suppress metronome marks?

[Graham King]

As a first-time lilypond (2.4.1) user, I'm very pleased at the ease of
input, and quality of output of the score.  But when it comes to fixing
the few remaining problems, I'm totally baffled (despite the
documentation).

Here's the first problem:
The two lines
        e2~ e8 r \tempo 4=90 r4
        \mark \markup{"Andante"}
produce the desired tempo change in the midi output, but result in
"Andante" (which I want to appear in the score) being overlayed by
"<quarternote> = 64" (which I don't want to appear at all).

I've tried all sorts of incantations that I don't really understand,
such as:
        \once \override MetronomeMark #'transparent = ##t
or 
        \set Score.metronomeMark = ##f
in various places in the input.  And I've had some abortive attempts to
switch Metronome_mark_engraver off.

Please could a guru step up and tell me what to do?  Thanks.