An invaluable resource for anyone interested in the early history of the House of York is Henry V and the Southampton Plot by T.B.Pugh. I don't always agree with Pugh's conclusions - he is quite hard on poor old Constance for example - but the book does contain almost all that is known about Richard of Conisbrough. Or as he perhaps should be called, Richard the Obscure.
Pugh believes that Richard was born in 1385, and although this contradicts what you may read elsewhere, I agree with him, because it makes sense. Richard was never knighted by his cousin, King Richard II, indeed did not achieve this status until 1406 at the hands of another cousin, Henry IV, at a time when bro. and sis. were both languishing in jail. Given that the Yorks were high in Richard II's favour it seems unlikely to me that a Richard born in say, 1375, would have got to 24 without being made at least a knight. Moreover that annuity I mentioned (worth a total of £333. 6s 8d when the Exchequer had money in it) did not start to be paid until 1395, a couple of years after his mother's death.
Pugh has an even more interesting theory - that Richard was actually the son of John Holland, not Edmund of Langley! Now, the evidence is strictly circumstantial, and as a writer of fiction I would not have dared to suggest it. But as the theory comes from an academic historian of Pugh's standing, I think the idea is worth considering.
Edmund of Langley left his younger son nothing. Although primogeniture ruled, and much land was entailed to the eldest son, it was customary by this era to make some provision for younger sons. John of Gaunt, for example, bought manors for the benefit of his Beaufort sons. Edmund did not leave Richard so much as a coin, a sword, or a second-best bed. So it is possible, and I put it no stronger, that Edmund believed Richard was not his son. It might also explain why Duchess Isabel left almost everything she had (bar the odd keepsake) to provide for Richard's future.