David thinks he's come to London to buy a walking frame, but he's about to meet actor Lionel Blair. At this point the prank goes wrong.
Michael McIntyre's Big Show returns soon for Series 4.
Britain's Got Talent star Alesha Dixon hands over her phone to Michael who sends a funny, festive text to her contacts.
After sliding down a chimney and onto the Big Show stage, John delivered a spine-tingling performance as the Christmas Unexpected Star.