[joeysteele]

The simple fact is that the real bulk of Labour voters and also Lib Dem voters will not in enough quantities support UKIP because of their EU stance of withdrawal at any cost.

It is in fact, Conservative voters who are the most volatile now as to the EU and they are the likely best hunting grounds for UKIP.
Even then though I doubt enough will turn to UKIP in a general election although I do think UKIP will do well in 2015,even picking up a few seats,not from Labour or the Lib Dems though but from the Conservatives.