Just FYI, here is Wikipedia's summary of Jane Austen's original fragment. It does seem plausible that Andrew Davies decided to allow his desire to add drama - and melodrama - to her story.
The novel centres on Charlotte Heywood, the eldest of the daughters still at home in the large family of a country gentleman from Willingden, Sussex. The narrative opens when the carriage of Mr and Mrs Parker of Sanditon topples over on a hill near the Heywood home. Because Mr Parker is injured in the crash, and the carriage needs repairs, the Parkers stay with the Heywood family for a fortnight. During this time, Mr Parker talks fondly of Sanditon, a town which until a few years before had been a small, unpretentious fishing village. With his business partner, Lady Denham, Mr Parker hopes to make Sanditon into a fashionable seaside resort. Mr Parker's enormous enthusiasm for his plans to improve and modernise Sanditon has resulted in the installation of bathing machines and the construction of a new home for himself and his family near the seashore. Upon repair of the carriage and improvement to Mr Parker's foot, the Parkers return to Sanditon, bringing Charlotte with them as their summer guest.
Upon arrival in Sanditon, Charlotte meets the inhabitants of the town. Prominent among them is Lady Denham, a twice-widowed woman who received a fortune from her first husband and a title from her second. Lady Denham lives with her poor niece Clara Brereton, who is a sweet and beautiful, yet impoverished, young lady. Also living in Sanditon are Sir Edward Denham and his sister Esther, Lady Denham's nephew and niece by her second husband. The siblings are poor and are thought to be seeking Lady Denham's fortune. Sir Edward is described as a silly and very florid man, though handsome.
After settling in with the Parkers and encountering the various neighbours, Charlotte and Mr and Mrs Parker are surprised by a visit from his two sisters and younger brother, all of whom are self-declared invalids. However, given their level of activity and seeming strength, Charlotte quickly surmises that their complaints are invented. Diana Parker has come on a mission to secure a house for a wealthy family from the West Indies, although she has not specifically been asked for her aid. She also brings word of a second large party, a girls' school, which is intending to summer at Sanditon. This news causes a stir in the small town, especially for Mr Parker, whose fondest wish is the promotion of tourism in the town.
With the arrival of Mrs Griffiths at Sanditon, it soon becomes apparent that the family from the West Indies and the girls' school group are one and the same. The visitors consist of Miss Lambe, a "half mulatto" rich young woman of about seventeen from the West Indies, and the two Miss Beauforts, common English girls. In short order, Lady Denham calls on Mrs Griffiths to be introduced to Miss Lambe, the very sickly and very rich heiress that she intends her nephew Sir Edward to marry.
A carriage unexpectedly arrives bearing Sidney Parker, the second eldest Parker brother. He will be staying in town for a few days with two friends who will join him shortly. Sidney Parker is around 27 or 28 and Charlotte finds him very good-looking with a decided air of fashion.
The book fragment ends when Mrs Parker and Charlotte visit Sanditon House, Lady Denham's residence. There Charlotte spots Clara Brereton seated with Sir Edward Denham at her side having an intimate conversation in the garden and surmises that they must have a secret understanding. When they arrive inside, Charlotte observes that a large portrait of Sir Henry Denham hangs over the fireplace, whereas Lady Denham's first husband, who owned Sanditon House, only gets a miniature in the corner — obliged to sit back in his own house and see the best place by the fire constantly occupied by Sir Henry Denham.