They don't need to regain the seats in Scotland; although I suspect many will slip back into the Labour camp when they realise that the SNP, like all small parties, is fairly toothless on its own, and they feel they have compensated the party for not voting for independence. No, the real change will be driven by the fact that any nation is going to get fed up of the existing government at some stage, and sooner or later enough voters drift into voting for giving the alternative a chance.