When Dreamers Cease to Dream — Barbara Bartholomew

A message sent by their android tutor QL47 from the future leads Jeanette Lacy, her brother Neil and their friend Jesse Lansden to travel in time again, for it says: 'WITHOUT YOU THE ALIENS WILL ALL DIE'. Knowing their friend Amy is amongst the aliens, they must act quickly. But first they must travel back to the most dangerous time of all - the time of the Guardians - to find QL47, because they need his advice to save Amy. But can they escape the fearsome Guardians a second time?