I've asked another of the Chew clan who has the Polebrook registers on fiche to check this out, and this is what she says:
William BELLAMY (d. 1778) married Lucy ROWELL; they had 4 known children, including another William BELLAMY d. 1773; the last was Adam BELLAMY, b. 1778.
Lucy BELLAMY nee ROWELL married John KNIBB, 17 Apr 1785.
They had 2 known children, including Lucy KNIBB [who later married Josrph CHEW], baptized 1786 as we know. Maybe Lucy junior was born before the marriage of John and Lucy senior, but my informant thinks the newspaper got the age of Lucy senior wrong:
Lucy CHEW nee KNIBB was buried 16 November 1826 [as we know] but aged 40, which ties in precisely with the date of baptism.
The William BELLAMY b. 1773 was probably the William BELLAMY who married Rebecca CHEW (b. 1775, sister of Joseph CHEW mentioned above), 24 April 1799.
That would make William BELLAMY and Lucy KNIBB half-brother and half-sister, not full brother and sister, and would also mean that Lucy was really born a KNIBB and not a BELLAMY.
Now my informant wants to know a bit more about John KNIBB! Can you oblige?

Geoff