Lilypond not finding plain Helvetica font

[Matthew Collett]

Following the 'official' snippet on how to change the default text font family, 
I try:

\paper {
 myStaffSize = #20
 #(define fonts
   (make-pango-font-tree "Times New Roman"
                      "Helvetica"
                      "Courier"
    (/ myStaffSize 20)))
}

\relative c'' {
 c4^\markup {
   roman: foo \bold bla \italic bar \italic \bold baz
 }
 c4_\markup {
   \override #'(font-family . sans) {
     sans: foo \bold bla \italic bar \italic \bold baz
   }
 }
 c2^\markup {
   \override #'(font-family . typewriter) {
     mono: foo \bold bla \italic bar \italic \bold baz
   }
 }
}

but this results in the upright sans-serif font always being bold, even when 
this is not requested.  Replacing "Helvetica" by "Helvetica Neue" gives the 
desired behaviour, but I do want plain Helvetica, for consistency with the text 
in which the Lilypond output is embedded.

This is Lilypond 2.14.1 on Mac OS X 10.6.8, and 'lilypond 
-dshow-available-fonts' does report the presence of 
family Helvetica
Helvetica:style=Regular
so the required font is present and recognised. 

Evidently it is a configuration error of some kind, but I have no idea where to 
start looking.

Best wishes,
Matthew