crossbencher

from WordNet (r) 3.0 (2006)

crossbencher
    n 1: a member of the House of Commons who does not vote
         regularly with either the government or the Opposition

from The Collaborative International Dictionary of English v.0.48

crossbencher \crossbencher\ n.
   a member of the House of Commons of Great Britain who does
   not vote regularly with either the Government or the
   Opposition.
   [WordNet 1.5]