The most recent account of Theresa, Lady Londonderry's career is to be found in H. Montgomery Hyde's The Londonderrys: A Family Portrait (London, 1979); the following extracts are taken from pp 63-69, 72-8, 83-5, 92, 94, 111-14, and 136-7.
'Lady Theresa Susey Helen Chetwynd Talbot, who married the 6th Marquess of Londonderry when he was Lord Castlereagh, was born on 6 June 1856 at Ingestre, the Talbot family seat in Staffordshire. Her father, then Viscount Ingestre, MP, succeeded his father as 19th Earl of Shrewsbury and 4th Earl Talbot in 1868. The Talbots were among the oldest families in the country, an ancestor, Richard de Talbot, being mentioned in Domesday Book, while the Earldom of Shrewsbury, dating as it did from 1442, ... made Theresa's father the Premier Earl of England. ...
Lady Theresa Chetwynd Talbot and Lord Castlereagh were engaged to be married in the summer of 1875. The match was arranged in the sense that their respective families approved of it; ... it is doubtful whether they were deeply in love with each other. ... [Following their marriage in October 1875], the Castlereaghs took Kirby Hall at Bedale in Yorkshire as a country house and also a London house at 76 Eaton Place. Their first child, a girl called Helen Mary Theresa, but always known in the family as "Birdie", was born on 8 September 1876 ... . On 13 May 1878 the Castlereaghs had a son, Charles Stewart Henry, who was born in Eaton Place. And, in the same month, Lord Castlereagh was returned after two expensive and unsuccessful attempts to get into parliament, the first for Durham in 1874 and the second for Montgomery in 1877, as Conservative MP for Co. Down in a by-election in which he defeated his Liberal opponent by a large majority.
Shortly afterwards, both being keen riders to hounds, the Castlereaghs acquired a house called The Hall at Langham, near Oakham, in the Cottesmore country. Here their second son and last child, Charles Stewart Reginald, was born on 4 December 1879. It was rumoured at the time, and it has been generally acknowledged since in the family, that Reginald's father was not Lord Castlereagh, ... but his wife's brother-in-law, Lord Helmsley [who died young in 1881]. ...
[Lord Castlereagh's succession to the Marquessate of Londonderry in 1884] naturally involved his taking his place in the Upper House, where he sat as Earl Vane, although he was customarily referred to by the superior title of his Irish peerage. Then, just as his father had added the surname Tempest to that of Vane, so the 6th Marquess by Royal Licence dated 3 August 1885 further added the original name of Stewart to that of Vane-Tempest for himself and his children, thus becoming Vane-Tempest-Stewart, although his brothers remained Vane-Tempest. ...
He was a wealthy man of property [even] by the standards of his times ..., and he and Theresa were particular favourites of the Prince and Princess of Wales, later King Edward VII and Queen Alexandra, whom they entertained in state [at their three country houses] no less than eight times between 1890 and 1903, six at Wynard, once at Machynlleth and once at Mount Stewart, not to mention sundry banquets and other parties at Londonderry House [Park Lane]. By all accounts the 6th Marquess was friendly, simple and unaffected, with a fine sense of public duty, and with none of his wife's [celebrated] hauteur. ...
"Lady Londonderry was a wonderful woman, with her masculine brain and warm feminine temperament", wrote ... Lady Fingall. "The best and staunchest friend in the world, she would back you up through thick and thin. In love with Love, she was deeply interested in the love affairs of her friends, and very disappointed if they did not take advantage of the opportunities she put in their way. She used to say of herself: 'I am a Pirate. All is fair in Love and War', and woe betide any one who crossed her in either of these." At her house parties at Wynard the bedrooms were conveniently allocated in the interests of her female friends and their lovers. She is easily recognisable as "Lady Roehampton" in Vita Sackville-West's novel The Edwardians. ...
Although she was the leading Tory political hostess of her day, Theresa Londonderry had friends among the Liberals, particularly [Sir William] Harcourt who often came to the house to discuss literature as well as politics; Theresa besides being widely read had literary pretensions of her own which were to find expression in an excellent short book on her husband's [collateral] ancestor the great Castlereagh, an abbreviated version of which had originally appeared in the Anglo-Saxon Review under the editorship of Lady Randolph Churchill. ...' Against this background, it is not surprising to find a great many important figures in literature, the arts, the army, the navy, the law and the church, and nearly every important figure in politics (particularly Tory politics) and High Society, of the period 1890-1919 among Theresa, Lady Londonderry's correspondents.