Past imperfect

Damian Barker is hugely wealthy and dying. He lives alone in a big house in Surrey. He has but one concern - his fortune in excess of 100 million and who should inherit it on his death. By the time he married in his late thirties he was sterile but what about before that? Had he sired a child? A letter from a girlfriend from these times suggests he did. But the letter is anonymous. Damian contacts someone he knew from their days at university. He gives him a list of girls he slept with and sets him a task: find his heir!