David,
Guardian,
"Contrary to the complacency among Labour’s campaign chiefs, until the last year – when Ukip was taking four or five Tory voters for every one Labour voter – in 2015 the Ukip share of the vote was higher in Labour-held seats than in Conservative-held ones."
https://www.theguardian.com/commentisfree/2015/may/23/labour-win-working-class-voters-ukip