A Trace of Memory

Description from goodreads:

When Legion meets a strange old millionaire called “Foster,” he doesn't know that his whole life will never be the same again. For “Foster” has a strange story to tell. Part of his story is contained in his diary, which not only describes things in the very recent past, but goes back for centuries. . .

Foster has hired Legion to help him regain his lost memory. Then both men are suddenly attacked by alien powers. They flee to Stonehenge in England where they discover a strange control chamber deep in the ruins.

They succeed in bringing down to earth an alien space ship which has been in orbit for thousands of years. Eventually they both become captives. . . on another planet.