Charles Hajdamach says in the book on page 271:
'These firms were retailers and dealers and the glass was made for them, presumably, at one of the London glassworks.
Some authorities give James Powell and Sons as the probable makers but there is no conclusive proof.'
And despite me unearthing the three court cases from 1851 and 1852, and the transcripts (evidence from the horse's mouth), and knowing now that they were not just retailers and dealers but also refiners, there is still no conclusive proof those pieces were made at James Powell and Sons unfortunately.
I agree that the book is fantastic resource to have, a great book.
m