The Wall — E.C. Tubb

Kerron and his business associates, Chang and Forrest, were three of the richest men on Earth. Now old and approaching the end of their days, they were desperate to prolong their lives. And there was one man who seemed to possess the secret of immortality-the mysterious Brett, a tall adventurer who had apparently lived for centuries. So the three men paid a fortune to the one man they believed could help them, But Brett had a dark secret, and he in turn made a bargain with them, They had to accompany him on the strangest and most dangerous journey ever undertaken - to the very centre of the Galaxy - beyond the Wall!