Gentleman Harry Simms, Flash, and the OED

[Robin Hamilton]

The OED, for FLASH, adj, Sense 5b, -- of the language spoken by thieves: 
Cant, slang -- has as its first citation:

    1747   Narr. Exploits H. Simms in G. Borrow Zincali (1841) II. iii. ii. 
129   They..began to talk their Flash Language, which I did not then 
understand.

Well, Henry Simms *was* hanged in 1747, and citation in question occurs as 
the epigraph to a chapter in George Borrow's _Zincali, or The Gypsies in 
Spain_.  However, I've never managed to find this form of words anywhere 
other than in Borrow's work, which suggests (it wouldn't be the first time) 
that Borrow composed the quotation from whole cloth.  It certainly doesn't 
occur in either the Old Bailey transcript of the trial of Henry Simms, the 
Ordinary's Account of Simms' life, or the text of an alleged Life published 
shortly after he died. ***

Has anyone ever found Henry Simms uttering the words which Borrow attributes 
to him, or is the OED sustaining a ghost here?

HDAS includes the OED citation, but encloses it in square brackets, which to 
my eye indicates a degree of scepticism.

Robin Hamilton

*** To give this its full title, _The life of Henry Simms, alias Young 
Gentleman Harry: From his birth, to his death at Tyburn, on June 17, 1747_ 

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