Slur crossing through accidental

[James Harkins]

I am at a complete loss what to do about this slur. It collides with the 
b-flat's accidental in a way that makes it hard to see what the slur is 
supposed to be.

\version "2.18.2"
\language "english"

\relative c {
 \clef bass
 \time 6/8
 bf8 ( d' ) r8
 bf,16 ( bf' ) d ( bf d bf )
}

I tried "\override Accidental.avoid-slur = #'inside," but this did nothing. 
I tried shifting the accidental to be closer to the notehead -- "\once 
\override Accidental.X-offset = #0.2" -- this does nothing. I tried using 
\shape but didn't find the magic numbers yet.

Suggestions? Thanks.

hjh