The danger was averted by the luck which threw Sir Edmund as a captive into the hands of Owen Glyndwr in the battle of Brynglas.

It was natural that Henry should refuse to allow Mortimer's kinsmen to ransom so formidable an enemy; but among these kinsmen Henry Percy ranked himself through his marriage with Sir Edmund's sister, and the refusal served as a pretext for a final breach with the king.
[Sidenote: Overthrow of the Percies] Percy had withdrawn from the Welsh war in wrath at the inadequate support which Henry gave him; and his anger had been increased by a delay in repayment of the sums spent by his house in the contest with Scotland, as well as by the king's demand that he should surrender the Earl of Douglas whom he had taken prisoner at Homildon Hill.

He now became the centre of a great conspiracy to place the Earl of March upon the throne.

His father, the Earl of Northumberland, his uncle, Thomas Percy, the Earl of Worcester, joined in the plot.

Sir Edmund Mortimer negotiated for aid from Owen Glyndwr; the Earl of Douglas threw in his fortunes with the confederates; and Henry Percy himself crossed to France and obtained promises of support.