Just been checking up on this. Apparently Denis was given a hereditary baronetcy in 1990, which meant that Maggie became Lady Thatcher at the same time. In 1992 she was made a life peeress in her own right then becoming Baroness Thatcher. Denis’s hereditary title has therefore already passed to Mark. It’s not clear why he was given it, possibly in recognition of his having remained married to Maggie for all these years?
Of course, she was Prime Minister when he was given it. Is that cronyism or what?
One of the enduring images from the 1980s series “Spitting Image” was the Mark Thatcher puppet emerging from a small flap in the door of No 10, and when the flap closed again it read “prat flap”.