There were actually at least 2 Lady Diana Spencers in the Spencer line (not counting Diana, Princess of Wales):
1)
Lady Diana Spencer (1710-1735), the daughter of Charles Spencer, 3rd Earl of Sunderland and Lady Anne Churchill (daughter of John Churchill, 1st Duke of Marlborough). This thread is dedicated to her. She had indeed been considered a possible wife for the Prince of Wales (mainly from the financial perspective - she had quite an impressive dowry), however George II voiced strong opposition to the union and, well, it never happened. She instead married John Russell, 4th Duke of Bedford and became Duchess of Bedford. She is indeed not in the direct line of ancestors of the Earl of Spencer, as she was the youngest daughter and child of the 3rd Earl of Sunderland.
This Diana was the sister of John Spencer, who was the father of John Spencer, 1st Earl Spencer (which makes her a great-great-great-------great Aunt of the current Earl Spencer).
2)
Lady Diana Spencer (1734–1808), the daughter of Charles Spencer, 3rd Duke of Marlborough and Honourable Elizabeth Trevor. She was the aunt of the aforementioned Lady Diana Spencer (she was the sister of 3rd Duke of Marlborough, 5th Earl of Sunderland, the second son of Charles Spencer, 3rd Earl of Sunderland - the father of the other Lady Diana; she was thus the aunt of John, 1st Earl Spencer). She was lady of the bedchamber to Queen Charlotte. Lady Diana was married twice: firstly, to Frederick, 2nd Viscount Bolingbroke (the marriage was annulled on the grounds of adultery, causing a huge scandal at the time and requiring an Act of Parliament for the annulment) and secondly (just two days after the annulment of her first marriage) to Topham Beauclerk. She had issue from both marriages. Both of her marriages were rather unhappy. One of Lady Diana's closest friends was Georgiana Cavendish, Duchess of Devonshire. She was quite a gifted artist and created illustrations for several noted books of the time.
She actually may be an ancestress of the current Spencers through the Poyntz line (Georgiana, Duchess of Devonshire was also from that line): although the 5th Earl Spencer (who was the son of Georgiana Poyntz and 4th Earl Spencer) died childless, I think there were other marriages between the Poyntz and Spencer families (or at least, the Spencers married the descendants of the Poyntz family).
