The Wizard in My Shed

When Rose (a completely ordinary girl) runs into Merdyn (a powerful warlock from the dark Middle Ages), at first she doesn't believe that this ridiculously dressed, and even more ridiculously spoken, chap could be a sorcerer. But it doesn't take long for her to realise that Merdyn could be useful to her. And so they make a deal.
In exchange for the visitor from the past brewing up a spell that will solve all her problems, she will teach him how to survive in the 21st century (for instance, that you NEVER drink from that porcelain “well” with a lid that's in the bathroom, or that mobile phones are not instruments of black magic), and help him find his magic wand so he can return to his own time.
What could possibly go wrong...?