Who was Sir John Mortimer?
Frankly, I have no idea! I've seen a family tree (In Felbrigg Hall, Norfolk, of all places) that made him the legitimate brother of Roger, 4th Earl of March and Sir Edmund Mortimer. However, I can find no other trace of him as such. He does not figure in the index of Wylie's History of England Under Henry IV, and if you're anyone at all in Henry's reign (and often if you're no one) you show up there.
Could he possibly have been an illegitimate son of the 3rd or 4th earl? Yes. Could he possibly have been a son of Sir Edmund Mortimer? Maybe, but almost certainly not by Catrin ferch Owain. Might he have been a collateral kinsman - for example a son of that Sir Thomas Mortimer who is a known illegitimate uncle of the 4th earl? Could be...
All we can say for certain is that the bloke was accused of treason in 1421 and executed (on the basis of an Act of Attainder - no trial) in 1424. And he was a 'kinsman' of the Earl of March.
If anyone knows more, please tell me.