Alex Sparrow and the Zumbie apocalypse

The Zumbies are on the rampage -- members of the Cherry Tree Lane zumba class are apparently dying and then mysteriously coming back to life! Alex, Jess and Dave have to put a stop to it before Alex's mum and nan join the living Zumba dead or there'll be no family Christmas. But why are the Zumbies curiously drawn to the Christmas lights? What does the Octopus sign mean? And who is the evil genius behind it all?