Pigot's 1823/4 Trade Directory has Benjamin Ellis, Watchmaker of Woodbridge Suffolk showing here.
http://homepages.rootsweb.ancestry.com/~mwi/suffkdir.txt
A previous Ellis, a later Ellis, or even a different Ellis may have made your Longcase Clock - Ellis may even have "bought it in" and sold it under his own name.
Good Luck
Peccavi
November 2008