Complex sentence: In 1939, the joint Senior Partners, Stanley Edgson and Douglas Overall lay the foundation stones at 77, Grosvenor Street, which was to become the firm's head office for more than 50 years.
Split into simpler sentences: In 1939, the joint Senior Partners, Stanley Edgson and Douglas Overall, lay the foundation stone at 77 Grosvenor Street. This address was the firm's head office for more than 50 years.