Do the judges vote again? Because if they do, I think Jill and Darren are safely home if they pull out all the stops:
If they come top = 3 points
Denise and Ian would surely come next = 2 points
Leaving Julian and Erin in last place = 1 point
And when the viewer vote comes around, it's likely that Denise and Ian will pick up 1 point, leaving Jill and Darren and Julian and Erin fighting over the 2 and 3 points. And whatever Jill and Darren pick up, Julian and Erin cannot better them nor draw level.
If they come top = 3 points
Denise and Ian would surely come next = 2 points
Leaving Julian and Erin in last place = 1 point
And when the viewer vote comes around, it's likely that Denise and Ian will pick up 1 point, leaving Jill and Darren and Julian and Erin fighting over the 2 and 3 points. And whatever Jill and Darren pick up, Julian and Erin cannot better them nor draw level.