story identification - Please Help identify a book featuring a robot, trolls, Merlin and stonehenge?

This sounds like the kids' book Sir Machinery where a wealthy hippy inventor type (American?) and his robot move to an old Scottish castle. A local crew of fairy creatures mistake the robot for a heralded knight (vague memories of a line about the fairies reading the robot's box like "MAC... HINERY. A fine Scottish name, although I've never heard of the clan." Arr arr arr.), and proceed to drag him and his owner into adventures in a magical world where trolls present an existential threat to the world, and Excalibur and Merlin figure prominently. I read it over 30 years ago, so can't quite recall Stonehenge, but that sounds right in line. The book cover is below.