Sir John Felton of Litcham in Norfolk was the son and heir of Sir Robert Felton, constable of the English-held Lochmaben Castle in Scotland and Scarborough Castle in Yorkshire, who was killed at Bannockburn in June 1314. John was already an adult when his father fell in battle; according to the Complete Peerage, he was knighted on 13 November 1310.  He was probably born sometime in the 1280s. John is perhaps most famous for holding out at Caerphilly Castle between November 1326 and March 1327 with Hugh 'Huchon' Despenser (b. 1308/9), eldest son and heir of Hugh Despenser the Younger and Eleanor de Clare, and as I pointed out in a recent blog post, he appears to have led Edward II's mysterious raid on Normandy in the late summer of 1326. Sir John Felton died in May 1344, according to this site; his heirs were his sons Hamo or Hamon (d. 1379) and Thomas (d. 1381) after his eldest son John the younger was murdered in or before 1334. 
At some point, I don't know when, John Felton joined the retinue of Hugh Despenser the Younger, lord of Glamorgan, and it's interesting to see that after Hugh's death John did everything he could to protect the life of Hugh's son and heir, holding out during the siege of Caerphilly for months on end even though he had never been officially appointed as the custodian of the castle (though he had sworn an oath on the Gospels to Edward II that he would keep the castle and everything in it safe).  On 5 August 1326, Edward II gave John a gift of £5 for his good service to Hugh.  Sir John Felton was one of the many Englishmen sent to Gascony in south-west France during Edward II's war against his brother-in-law Charles IV of France in 1324/25, after Charles and his uncle Charles de Valois invaded Gascony in the summer of 1324. John acted as custodian of the castle of Saintes from October 1324 until the end of 1325. 
Dozens of the letters Hugh Despenser the Younger sent to men in Gascony during what is known as the War of Saint-Sardos of 1324/25 survive today, thanks in large part to Hugh's habit of keeping drafts of his own correspondence. They reveal a great deal about Hugh as a person, his enormous influence over Edward II and the way he wasn't shy about coupling himself with the king of England as though they were co-rulers, his astonishing arrogance, sense of entitlement and self-importance, and the fawningly obsequious way he was addressed by just about everyone, even men who were superior to him in rank, such as the earls of Kent and Surrey. See my book Hugh Despenser the Younger and Edward II: Downfall of a King's Favourite for numerous examples of all of this; my absolute favourite bit is Hugh telling John Travers, constable of Bordeaux, that "as a result of your good conduct, the king and ourselves may discuss continuing our goodwill towards you." Oh, the self-important haughtiness.
Despenser and John Felton exchanged eight letters in 1324/25 which still exist. In 1954, Professor Pierre Chaplais printed the letters, in the original Anglo-Norman, in his astonishingly useful volume The War of Saint-Sardos (1323-1325): Gascon Correspondence and Diplomatic Documents, and the originals are in the National Archives. John Felton's first letter to Hugh Despenser was sent from Bordeaux on 17 October 1324, and the last letter, or at least the last surviving letter, from Hugh to John was sent around the end of April 1325.
John Felton's letters to Hugh are highly illuminating about the reality on the ground in Gascony, and make it apparent that John was one of the royal chamberlain's informants (see my post about Hugh's informants and spies) and had been probably been handpicked as such. In his first letter of 17 October 1324 and again on 24 November, Felton warned Despenser that a "wicked faction", maveyse covine, was gathering around Edward II's half-brother and lieutenant in Gascony. This was Edmund of Woodstock, earl of Kent, who in September 1324 had, perhaps rather tamely, surrendered to Charles, count of Valois, who was the uncle both of Charles IV of France and of Edmund himself (being the older half-brother of his late mother Marguerite of France). This faction was, according to Felton, putting all the blame for Kent's defeat on Hugh Despenser for failing to send enough money and soldiers to Gascony, and furthermore, they were "plotting as much evil against you [Hugh] as they can". Felton was excellent at communicating to Despenser precisely how many men and horses he thought were required in various situations, and at telling him frankly that Gascony would be lost to the English unless more men, victuals and money were sent there immediately. John stated on 24 November 1324, rather plaintively, that he had only received wages for forty days and that to raise enough money to provision the castle of Saintes, he had had to sell all his own horses (joe vendus mes chyvaus memis tous pur vitayller le chastel). 
Hugh Despenser the Younger must have been a very difficult, and genuinely quite terrifying, man to work for; in 1322/23, he took against his own adherent Sir John Inge, sheriff of Glamorgan, and imprisoned him and all the members of his council in Southwark, despite Inge's years of loyal and excellent service to him. Thomas Langdon, one of Inge's councillors, died in Hugh's prison. Hugh wrote to Inge, seemingly casually in the middle of one of his long, hectoring letters in c. October 1322, that "we trust you more the more you advise us, but we are very worried about having some reason for which we might be prepared to harm you" (see my Downfall of a King's Favourite). Sir John Felton must surely, therefore, have been intensely relieved when Hugh told him on 4 October 1324 how much he appreciated his diligence, loyalty and good conduct, and that he, Hugh, felt "much great joy", molt grand joie, when he heard good news of John, as if he were his own cousin (a taunt come si vous fuissez nostre cousin germain). John sent a reply to this letter on 3 December 1324, two months later, and thanked Hugh for "your amiable letters" and for telling him that "you are glad when you have good news of me". He promised that he would work as hard and as loyally as he possibly could to serve Edward II and Hugh. A few months later in April 1325, Hugh told John that the king "has greatly given you his heart" (vous ad done grandement son queor) because he was so pleased with John's service. 
It seems to me that John Felton's letters to Hugh Despenser the Younger were less servile than other men's; they began simply "To his very honourable lord, honours and reverences" and ended "Lord, may God grant you a good and long life." His warnings to Hugh Despenser to be wary of Edmund, earl of Kent, and his "wicked faction", interest me very much. Kent himself sent five letters to Hugh the Younger while he was in Gascony, addressing him repeatedly as "dearest and beloved nephew" even though Hugh was a dozen years older than he (Hugh's wife Eleanor de Clare was the eldest granddaughter of Edward I, Kent's father). Hugh did not reciprocate this familiarity but addressed Edmund in conventionally polite though distant terms, except in one letter of 21 June 1325, when a letter from Hugh to the earl was written in such overblown and exaggeratedly obsequious fashion that I feel certain he was being sarcastic.
Sir John Felton survived Hugh Despenser the Younger's and Edward II's catastrophic downfall in 1326/27, and was pardoned in March 1327 for holding Caerphilly Castle against Queen Isabella and for "committing depredations" in Normandy. I've never found evidence that he took part in any of the plots to free the former king in 1327 and again in 1330, though on 14 October 1326 Felton was appointed to defend "the March of Wales" against the invading forces alongside one other man: Donald of Mar.  Donald, a close friend of Edward II despite being Robert Bruce's nephew, most certainly was involved in the plots to free Edward, and in 1329/30 offered to lead a massive Scottish army into England to help his friend, even years after Edward's supposed death. Given that John Felton had been so close to Hugh the Younger and helped to save his son's life after his death, and that he was still loyal to the king a few weeks after the invasion and was associated with Donald of Mar, it doesn't seem impossible that he might have been involved in the attempts to free the former king from captivity.
1) Complete Peerage, vol. 5, pp. 289-92.
2) Calendar of Inquisitions Post Mortem 1377-84, nos. 339-43; CPR 1330-34, p. 552.
3) Calendar of Fine Rolls 1319-27, p. 430.
4) Society of Antiquaries of London Manuscript 122, p. 79.
5) Pierre Chaplais, ed., The War of Saint-Sardos (1323-1325): Gascon Correspondence and Diplomatic Documents, p. 274; Calendar of Patent Rolls 1324-27, p. 3.
6) Chaplais, War of Saint-Sardos, pp. 83, 106-7.
7) Chaplais, War of Saint-Sardos, pp. 79, 111, 115, 173-4.
8) CPR 1324-27, p. 332.