Later versions of the prop had more efficient wheels ( from shopping carts , according to a Blue Peter episode ) and were simply moved by the seated operators ' feet , but they remained so heavy that when going up ramps they often had to be pushed by stagehands out of where the camrea could not see . Later versions of the prop had more efficient wheels and were once again simply propelled by the seated operators ' feet , but they remained so heavy that when going up ramps they often had to be pushed by stagehands out of camera shot .