Easy to ID the Earl of Rosebery in 2006 (the year the plaque was erected): Neil Archibald Primrose, 7th Earl of Rosebery (1929 - ) descended from Hannah, but, according to The Peerage his wife, née Alison Mary Deirdre Reid (1931 - 1978) had already died and the Earl does not seem to have married again. He is, 2018, still alive so his son Harry has not yet, and had not in 2006, inherited the title. So we could not imagine who was titled "Countess of Rosebery" in 2006.
We almost gave up at that point but carried on searching. We found the Getty Images photo of a Countess of Rosebery, dated 1998, and she looks like she could have been born in 1931. We then found a connection with the Caledonian Ball which led us to ..... the magazine for the 2010 Caledonian Ball, in which Lady Caroline Dalmeny, the Chairman (sic) refers to her husband Harry (the heir to the Rosebery title) and informs that her mother in law, the Countess of Rosebery, would be joining her party for the evening. That convinces us that The Peerage must have it wrong, and that the Countess was, at least in 2010, still alive enough to be kicking up her heels at the Caledonian Ball.
Assuming that is correct: The Countess of Rosebery who unveiled the plaque in 2006 married the 7th Earl in 1955 and goes by her middle name of Deirdre.
