In the early autumn of 1327, a group of Welsh knights plotted to release the former Edward II from captivity at Berkeley Castle - which appears to have been unrelated to the plot of the Dunheved brothers and their allies to free Edward that summer. The ringleaders were Rhys ap Gruffydd of South Wales, lord of Narberth, sheriff of Carmarthen and formerly a squire of Edward II's chamber (see here for more about him), and Sir Gruffydd Llwyd of North Wales, lord of Dinorwig and Tregarnedd, sheriff of Anglesey and Merioneth. Rhys (with half a dozen of their allies) fled to Scotland after the failure of the plot, while Gruffydd was imprisoned at Caernarfon Castle for eighteen months. 
Edward II's loyal Scottish friend Donald, earl of Mar, Robert Bruce's nephew, appears to have been deeply involved in the plots to free Edward from Berkeley in 1327. Donald - captured in 1306 as a child by Edward I and imprisoned at Bristol Castle - remained totally loyal to Edward II throughout his reign and only returned to his homeland at the time of Edward's capture in November 1326. In the summer of 1327, he led one of the columns of his uncle Robert's army against the new regime, and Queen Isabella and Roger Mortimer evidently considered him a great threat: in July and August 1327, they ordered the arrest of two of his supporters in Staffordshire merely for sending letters to him, and also ordered the arrest of Richard le Brun, former mayor of Chester, for adherence to Donald. (The Dunheved brothers, perhaps not coincidentally, were in Chester in early June 1327.) In the summer of 1327, Donald's adherents were said to be gathering in the Marches "to do and procure the doing of what evils they can against the king [Edward III] and his subjects" and to have returned from Scotland "to do what mischief they can to the king and his realm." 
What's interesting is that Donald of Mar, Rhys ap Gruffydd and Gruffydd Llwyd's son Ieuan (future governor of the Channel Islands) were also involved in the earl of Kent's plot of 1329/30 to free his half-brother Edward - supposedly dead since September 1327 - from Corfe Castle.  William Melton, archbishop of York, sent a messenger to Donald to inform him that Edward was alive; Donald duly promised Melton that he would come to England with an army of 40,000 men when instructed by the archbishop.  (Rather a large army to free a dead man, you might think.) Rhys ap Gruffydd met other enemies of Roger Mortimer and Isabella in Paris and Brabant - whose duke was Edward II's nephew - who were also plotting an invasion of England. On 8 August 1330, Roger Mortimer appointed himself, as justice of Wales, to arrest and imprison any followers of Rhys, who was said to be "propos[ing] to enter the realm with a multitude of armed men"; it was said that many people in Wales were "of his confederacy." 
This planned invasion never took place, but caused panic in England, where Roger Mortimer and Isabella were by now deeply unpopular: the Brut chronicle wrote that the country was "ful sore adrade, and almost destroiede," and "bigan the communite of Engeland for to hate Isabel the Quene...".  In mid-July 1330, Mortimer and Isabella ordered all the sheriffs in the country to array knights, squires and others who bore arms; they should prepare themselves as speedily as possible "to set out against certain contrariants and rebels who lately withdrew secretly from the realm and who have assembled a multitude of armed men in parts beyond the sea and have prepared ships of war and other things and who propose entering the realm to aggrieve the king and his people." Other arrays were ordered in late July and August against "certain rebels who lately withdrew from the country by stealth." 
As well as Edward II's good Welsh and Scottish friends, there were plenty of English men keen to free him from captivity. In 1330, they included the bishop and mayor of London; William la Zouche, lord of Ashby, who had besieged Hugh Despenser the Younger's castle of Caerphilly in 1326/27 with Hugh's son inside; Malcolm Musard; William Aune; Giles of Spain; and many dozens of others.
1) See my previous post, which lists the sources for this paragraph.
2) Gesta Edwardi de Carnarvon Auctore Canonico Bridlingtoniensi, in W. Stubbs, ed., Chronicles of the Reigns of Edward I and Edward II, vol. ii, p. 96; H. Maxwell. ed., The Chronicle of Lanercost 1272-1346, pp. 256-257; Calendar of Close Rolls 1327-1330, pp. 142, 157, 212; Calendar of Patent Rolls 1327-1330, pp. 139, 180-181, 183, 191.
3) E.M. Thompson, ed., Adae Murimuth Continuatio Chronicarum, p. 256; Calendar of Fine Rolls 1327-1337, pp. 169-170.
4) Ian Mortimer, 'The plot of the earl of Kent', in his Medieval Intrigue: Decoding Royal Conspiracies, p. 161; Seymour Phillips, Edward II, p. 567.
5) Murimuth, p. 256; Close Rolls 1330-1333, p. 51.
6) F.W.D. Brie, ed., The Brut or the Chronicles of England, vol. 1, p. 257.
7) Close Rolls 1330-1333, pp. 147, 151; Patent Rolls 1327-1330, pp. 544, 563, 570-572.