Complex sentence: In 1993, he became group finance director of Grand Metropolitan, the food and drink giant but, when it merged with Guinness to form Diageo, he lost his job to his counterpart at Guinness.
Split into simpler sentences: In 1993, he became Group Finance Director of Grand Metropolitan, the food and drink giant. When "Grand Met" merged with Guinness to form Diageo, he lost his job to his counterpart at Guinness.