Might need to tidy up a wider area. Bad connections play havoc with dates.
Hist Parl on Sir Robert Constable d 1401, father of Sir Marmaduke
Thinks he was born about 1353, same date as given by ODNB. Not later, though I don't see why it couldn't be earlier.
Says his wife Margaret Skipwith was the widow of Alexander Surtees d 1380. If Sir Marmaduke was her son, he can't have been more than 23 when he died in 1404 leaving 4 young sons. Just possible, but no slack.
Kills off Foster's statement that he was living 29 Edw III 1355, which came from the 1612 Visitation manuscript.
Hist Parl on Katherine's brother Sir Thomas Cumberworth
His will, 1450 pr 1451
He's a bit of a puzzle - apparently of age by 1401, but doesn't seem to have taken up public duties until 10 years after his father died. Hist Parl guesses he was very young, but in 1436 he got an exemption for his great age - after which he carried on doing the rounds until he died anyway.
My best guess for his DoB would be 1375-80. This is consistent with his sister being of similar age to her husband.
So this idea that Sir Marmaduke and Katherine had a daughter Elizabeth b 1363
is dead from all directions. It would also mean Katherine would have had to be about 60 when her grandfather Sir William Ergham died. Appears to cite Richardson, but Richardson doesn't actually say that. He only says Stephen Thorpe jr's mother was "Elizabeth daughter of Marmaduke Constable".
Katherine's grandparents are also getting messed up
There was a Sir William Ergham who married Sibyl FitzHenry and left her a widow in 1347 (CP2 Vol 5 p. 415). But Katherine's grandfather was a later Sir William Ergham, who married Katherine Cressy and died about 1404, leaving 2 daughters and coheiresses, Julian and Sibyl. Julian married John Aske, of Aughton, and died sp (he.then married Judge Gascoigne's daughter).
So where does this leave Thomas Hercy? This Katherine wasn't the wife he had in 1397, but she was widowed young and could have married him later.
On the other hand, Marmaduke's 2nd son was called John, b abt 1400. He could have married Hercy's widow or his daughter.