Space foot note away from page number at bottom of page

[Nick Payne]

In the following, how can I increase the spacing between the footnote 
and page number? Trying the \override FootnoteItem #'Y-extent doesn't 
make any difference:

\version "2.15.25"

\paper {
    print-page-number = ##t
    print-first-page-number = ##t
    oddHeaderMarkup = \markup \null
    evenHeaderMarkup = \markup \null
    oddFooterMarkup = \markup {
        \fill-line {
            \on-the-fly #print-page-number-check-first
            \fromproperty #'page:page-number-string
        }
    }
    evenFooterMarkup = \oddFooterMarkup
}

\relative c' {
    \override FootnoteItem #'Y-extent = #'(-4 . 4)
    \footnote #'(0.5 . 5) #'NoteHead \markup { Now is the time for all 
good men to come to the aid of the party }
    c4 c c c
    \repeat unfold 600 { c4 }
}

Nick