How could Sir John Campbell, K.C.T.S., afford to live on Charles Street, Berkeley Square?

As I've been looking at records of the life and career of Sir John Campbell (1780 to 1863), many times I've wondered about money. I first found him because I've been looking at  Charles Street, Berkeley Square in London, a prestigious address with many poobahs as neighbours, and that's where he lived in the last decades of his life. My lingering question was, how could he afford it?

Sir John's father, William Campbell, was a Commissioner of the Navy, a high-ranking civil servant, but not necessarily a wealthy man once the benefits of his office (notably, a house) were removed.

Sir John's two sisters married two brothers of the Onslow family, who themselves have an illustrious pedigree. However, any Onslow family wealth and land would have bypassed Elizabeth and Marianna Onslow nee Campbell and passed to the male heirs.

Sir John did enjoy some hospitality from his sister Elizabeth and her husband, Reverend George Onslow. In the 1841 census, we find Sir John and "Elise", whom I assume to be Sir John's daughter Elizabeth, with the Onslows at their family home, Dunsborough House in Send, Surrey. I've taken this to be either  the normal reciprocal visiting among family members or a temporary residence for Sir John and Elizabeth. At some point during or after 1834, Sir John returned from his stay as a prisoner of war in Portugal. During his absence, his only child Elizabeth may have been sent to stay with the Onslows, and perhaps Sir John joined her there.

Website for The Wey Valley, with a picture of Dunsborough House and interesting history about the villages of Send and Ripley

So, I've ruled out inherited wealth, but I should look for the will of Sir John's father and also of Sir John's siblings, just in case there is a pot of gold somewhere. It's unlikely any siblings transferred any wealth to him. I think each had a family of his or her own to care for.

Sir John's first wife, the young Portuguese lady Dona Maria Brigida de Faria e Lacerda has a noble-sounding name and it wouldn't surprise me if her family had a prominent position in Portugual. However, she married out of her society and went to England, where she died young. Sir John was persona non grata in Portugal after supporting the losing side in the War of the Two Brothers (and being a noted prisoner of war following it). Also, I have always had the impression that being an army officer in his day was not usually a way to get rich. I suppose perhaps there were occasional opportunities for plunder, but he hardly seems to have been in the right place and time for that, at least not at the latter stage of his Army career.

Having written off these various sources, I took a closer look at who owned 51 Charles Street and how Sir John came to live there at all.