Interesting article in British Wildlife Vol 20, no 4 april 2009 237 -244. The Polecat in Britian -continuing recovery.

Amongst other things information there about differences between true Polecat and interbreeding, (Polecat-ferret).

Havn't checked VWT website but they feature heavily in the article, presumably some information on their website about pure polecat and interbreeding.