How to shift trill to below phrasing slur?

[Andy Bradford]

Hello,

I have a small section of music that has a trill that Lilypond places on
top of a phrasing  slur, but I would like it  beneath (between the staff
and the slur).

I've tried to use:

-\tweak Y-offset #-3 \trill

And it doesn't seem to have any effect at all (even though modifications
to the X-offset do). I've also looked at the avoid-slur property but I'm
not sure I know how to use  it (and attempts I've made haven't seemed to
make a difference).

Here's MWE:

\version "2.18.2"

\score {
  {
    \time 12/8
    \relative c' { 
      a8 b\( c a b c d e f d e f 
      g a b g a b c2.
      d2.\trill \grace { c16 d } e2.\)
    }
  }
}

Thanks for any suggestions.

Andy