From the Medical Heritage Library, new information about Knighton's early medical career – definitely before 1813, when he received his baronetcy, and probably between 1798 to 1803, successful but difficult years when he was in private practice in Plymouth.  

In 1815 the Berks and Bucks Vaccine Institution published a paper to promote vaccination with cowpox as a safer alternative to inoculation with smallpox. A footnote said that some years earlier the then Dr Knighton took matter from the arm of a girl who had developed cowpox following vaccination, and inserted it into one of his patients where it 'produced the disease in a satisfactory manner'.  

This account is entirely plausible. The girl was the younger daughter of a Plymouth physician, Thomas Stewart, who vaccinated her in early 1801 when Knighton was still practising in Plymouth. The young Dr Knighton did not have happy memories of these years; his comment on a bruising incident with unnamed local colleagues was, 'Everything that is wrong punishes itself'. 

In 1803 Knighton left Plymouth for London, and a year later Stewart wrote up the results of vaccination on his own children. If he had ever collaborated with the young doctor who had since left the area, he didn’t bother to mention him.

William Knighton started his London practice in 1806; by 1812 he was said to have connections with almost every family of distinction. During the next ten years his patients included Leigh Hunt and his family; Richard Wellesley, 1st Marquess Wellesley, introduced by Wellesley's mistress; the family of John Wilson Croker; Harriet Canning,  George Canning's young daughter;  Lady Charlotte Grimston, Countess of Verulam, sister to Lord Liverpool;  Henry Gage, 3rd Viscount Gage; the  Countess of Warwick (portrait); Byron; and novelist Catherine Stepney. After Princess Charlotte's death he spoke to several of her close friends, including mother and daughter Lady Anne Smith and the Marchioness of Worcester, to assure them that Croft's conduct had been correct. So when Knighton became a full-time courtier in 1822 he was already a household name in the best households. 

On entering the King's service Knighton gave up his medical practice, but he did not give up medicine. Artist and fellow Devonian Samuel Prout, incapacitated by headaches, overcame his embarrassment at approaching his former physician and asked for a prescription that Knighton had successfully prescribed for him many years earlier. Harriett Arbuthnot was also grateful for a prescription from Knighton, despite calling him 'a great rogue and a blackguard'. On retirement to the country after George IV's death Knighton gave medical advice to anyone who asked. 

Knighton's treatments were conservative. He favoured bleeding, recommending his wife to apply not more than half a dozen leeches to her temples if she felt headachy or bilious. He disapproved of cold-water bathing as a remedy. He enjoyed his food, but preached and practised moderation. When he travelled to Spain with Richard Wellesley he took a portable bed to avoid lice, and warned his party of the dangers of bad water. On occasion, perhaps believing that hope was his only remaining remedy, he gave reassurance that he must have known was ill founded. But it was also said that he was able to gain control over his patients' minds, much to their benefit.

Knighton, Croft, and Princess Charlotte: After publication I discovered new material. It's discussed towards the end of the comments here.

Between September 1796 and January 1797, when Knighton was nineteen going on twenty, he undertook the rite of passage between apprenticeship and medical practice, and left Devon to 'walk the wards' under a prestigious medical man at a London hospital. He paid a fee to become a pupil of surgeon Henry Cline at St Thomas's Hospital, paid more fees for lectures by other medics, and spent his Saturday evenings at the Guy's Physical Society which was open to staff and students of St Thomas's and Guy's hospitals.

Each week a Society member reported a case that he had observed, so speakers were usually experienced medical men. On 17 December 1796, however, the floor was given to one of Knighton's fellow students, Hall Overend, who dispassionately reported symptoms he had experienced after cutting his finger in the dissecting room ten weeks earlier.  

The wound was so slight that Overend had ignored it, but within two days it was ulcerated, his whole arm was stiff and he had a swollen gland in his armpit. He healed the wound but the pain and swelling increased. A week later he consulted Astley Cooper, who was another of Knighton's lecturers and a former pupil of Cline. During the next two weeks Overend suffered throbbing pain, delerium and debility. The swelling hardened and filled his armpit, at one stage extending to his nipple. Cooper prescribed leeches to reduce the swelling, a succession of purgatives to flush out the infection, opium for the pain, and incisions in the swelling to remove pus. To discourage further swelling he placed Overend on an antiphlogistic regimen of simple diet and no alcohol.  

At the time of speaking Overend had not yet fully recovered, but he survived to return to Sheffield where he had been apprenticed. There he established a successful practice as a surgeon apothecary, founded his own school of anatomy and medicine, and supported the creation of a second school which is now part of Sheffield University.

Here is a bust of Hall Overend. http://www.picturesheffield.com/frontend.php?keywords=Ref_No_increment;EQUALS;s07723&pos=2&action=zoom 

Astley Cooper in 1825, nearly twenty years after he treated Hall Overend.
Wikimedia Commons http://en.wikipedia.org/wiki/File:Astley_Paston_Cooper_2.jpg

John Cary's map, c1812, showing St Thomas's and Guy's hospitals south of the Thames on the approach to London Bridge http://www.foldingmaps.co.uk/cary.jpg

George IV’s death in June 1830 released Knighton from royal duties that had sapped his health and spirits. By July he was planning his retirement, and in September he sold his London home, 9 Hanover Square. The purchaser was Charles Locock, an obstetrician in his early thirties to whom Knighton had once referred patients.

More than twenty years earlier, as a provincial doctor making a second attempt to 
establish himself in London, Knighton had bought the property from Robert Hallifax, one of the physicians-in-ordinary to George, then Prince of Wales. In contrast Locock, who enjoyed the patronage of royal surgeon Benjamin Brodie and respected obstetrician Robert Gooch, both of whom Knighton knew well, brought a thriving practice to 9 Hanover Square.  Nevertheless Knighton and Locock had much in common. Both had attracted mentors who guided their early careers. As their practices grew, both noted a rapid, exponential increase in lucrative connections. And both were befriended by slightly older women who became their confidantes (more about that in a later post).

Robert Hallifax died in 1810, and two years later Knighton was appointed physician-in-ordinary in his place.  In 1831 Locock became physician accoucheur to William IV’s consort, Queen Adelaide, whom Knighton attended in 1820 when she was Duchess of Clarence. In 1840 Locock became physician accoucheur to Queen Victoria.

***

I shepherded the bio through publication, shuffling commas and tweaking headings, ignorant of letters at Westminster City Archives in which Locock discusses Knighton’s practice. How important is stuff like that in a biography of a physician? How many eons did I waste looking for it in the wrong places? What of the trillion hours I wiled away in Westminster City Archives dotting and crossing insignificant 'i's and 't's?  Why did it not occur to me to check out the Locock papers at the Wellcome Library? Now I find there's even a biography of the wretched man.

***

Hanover Square is south of Oxford Street. You can just about make out No 9, second down from the north-east corner.  http://www.motco.com/Map/81005/Sale/imagetwo-a.asp?Picno=81005860&title=+return+to+your+previous+screen+close+this+window+%2D+click+on+the+X+in+the+top+right+corner%2E 

Sources:

• G T Bettanny, revised by Anne Digby, ‘Locock, Sir Charles, first baronet (1799-1875), obstetric physician’, Oxford Dictionary of National Biography http://www.oxforddnb.com/view/article/16915 [accessed 9 June 2013, free to members of UK libraries] 
  
• Russell C Maulitz, ‘Metropolitan medicine and the man-midwife: the early life and letters of Charles Locock’, Medical History, Vol 26, Issue 1, Jan 1982, pp25-46 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1139111/?page=1 [accessed 9 June 2013]
  
• Morning Chronicle, London, 11 December 1820 and many subsequent dates
• London Metropolitan Archives: Middlesex Deeds Registry MDR 1834/8/247 and MDR 1834/8/248
  
• Westminster City Archives, London: Poor, Watch and Paving Rates Collector’s Book, MF451