Busbeater's Magnificent Adventures in Time — Stephen Price

A reluctant 12 year old Gerald Jones is sent to Busbeater Mansions, his uncle's grand but dusty old country house for the summer holidays where he expects to be very bored for weeks on end. But then one rainy day a mysterious stranger arrives beating urgently on the door; whoever he is, he has important and surprising news. It turns out that Busbeater Mansions has a massive secret; a Time Machine lolling about upstairs. With the evil Baron von Achtung in hot pursuit Gerald, Uncle Busbeater and the rest of his wildly eccentric relations and friends set off on a magnificent adventure across space and time landing, wrongly, at the court of tyrant King John on the day before he agrees Magna Carta. Can they escape the King's clutches, get to the right place and time, defeat the Baron, save the day and get home safely, via the moon obviously, all without changing history? There are many, many moments when Gerald is not so sure.