Papers concerning wardship and marriage of Nicholas son of Nicholas Bacon, mention Eustace Darcy and John Rous as committees. The papers are from 1641-1647 and consist of 9 parchments and papers and might state and prove the relationship between the Eustace Darcy and Nicholas Bacon who died in 1641. His daughter Anne was married to John Rous.
The papers of Nicholas Bacon of Gillingham, Norfolk state the existence of Eustace Darcy and John Rous as the executors of Nicholas Bacon. They took over his executorship of Bacqueville Bacon of Hockham who died in 1635. Bacqueville (Bacon-6101) was the brother of Nicholas (Bacon-3495).
It would be interesting to get hold of scans of the above documents. They might state the relationship between Eustace Darcy and Nicholas Bacon. If so, they might have been Burke's source.
The papers from 1620 concerning a property in Colchester, Essex, mention Eustace Darcey and his wife Elizabeth as defendants and John Norton as plaintiff. This document might state the property.
According to British History Essex one John Norton was a bailiff and mayor of colchester in the years of 1620, 1624 and 1630.
The papers from 1603-1625 concerning a court case with Eustace Darcy as plaintiff and Edward Suliard as defendant, suggest a connection which turned into a speculative but still interesting study when I found the following:
One Eustace Sulyard of Runwell, Essex (Sulyard-16) married Margaret Foster (Forster-609). They had a daughter Margaret Sulyard (1539-1586) who married Thomas d'Arcy (1538-1586) and a son called Edward Selyard (1535-1610). Now, Thomas d'Arcy II of Tolleshunt and Margaret Sulyard apparently had a son called Eustace Darcy.
Could it be that this Eustace Darcy had a dispute with family member Edward Suliard ? If they are the same Eustace, one could work towards creating a profile for Eustace Darcy as the son of Thomas Darcy (Darcy-320) and father of Margaret Bacon (Bacon-545).
The will of Thomas Darcy dated 14 November 1593 and proved 1594 apparently does not mention his siblings but the Eustace Darcy who witnessed the will was probably his brother.
Thomas Darcy on geneanet mentions a will of father Thomas Darcy (1583-1586) in which Eustace Darcy is mentioned as one of his children. It stated that he may be the son referred to as "Estras". It is furthermore stated that Estras Darcy is younger than 21 (=therefor born after 1573).
And the archived will refers to a pedigree of the Darcy's which shows Esdras as the 3rd child of Thomas and Margerett.
Essex Institute Historical Collections Volumes 17-18 has on page 89 a Norton pedigree which shows that John Norton, son and heir, of Bury St. Edmunds was married to one Elizabeth who afterwards married Eustace Darcy of Norwich Esq. On page 94 it mentions Eustace Darcy of Norwich Esq. and his wife Elizabeth in the probate of John Norton of Colchester, Co. Essex, gentleman, one of his majesty's Bailiffs of the Town, probate 31 Aug. 1631 proved 5 Nov. 1631.
