I was wrong (no change there)!
Here's what I got from Baroness Thatcher's Wikipedia entry:
In 1992, Margaret Thatcher was raised to the House of Lords by the conferment of a life peerage as Baroness Thatcher, of Kesteven in the County of Lincolnshire, upon her. She did not take a hereditary title, as she had recommended for Harold Macmillan, later Earl of Stockton, on his ninetieth birthday in 1984. She has explained that she thought she hadn't sufficient means to 'support' an hereditary title.
So I'm at a loss as to why Mark Thatcher is a 'Sir'.