Hi Geoff
I did some work on these Chews a few months ago in connection with some Rogers research I was doing. It was a bit messy!
The Chews were from Southill, Beds, although the Chews who were born in Bourn had no Chew blood in them.
Elizabeth Rogers, who was baptised at Bourn on 28 Dec 1777, moved with her parents John and Elizabeth (Darlow) Rogers, to Biggleswade, Beds c1778, and then to Southill c1786 where she married Joseph Chew on 31 July 1797, and had two children, Joseph and Elizabeth, both baptised 21 Jul 1799. The Rogers family together with their daughter Elizabeth Chew returned to Bourn somewhere between 1801 and 1805.
Elizabeth (Rogers) Chew had six illegitimate children in Bourn between 1805 and 1816 - John 1805, Catherine 1807, Isaac 1811, William 1812, Jane 1814, Robert 1816 - where the parish register names the father of them all bar John and Isaac as William Saywell. In 1818 she married William Saywell. I cannot trace a burial of her husband Joseph Chew, but it's possible that she didn't marry William Saywell until 1818 as her husband was still alive.
The link between Bourn and Southill is provided by the Bourn 1851 census - William Saywell 77 b Bourn; Elizabeth Saywell 74 b Bourn, William Saywell 39 son b Bourn, Elizabeth Chew 53 dau in law (ie step daughter) unmarried b Broom Beds (Broom is a hamlet of Southill)
I didn't look at the Chews or Saywells in Bourn, but I can see from the transcript that Isaac was baptised in Bourn on 11 April 1837, the son of Robert, and Katton appears from censuses as being the daughter of Robert's brother Isaac, so as you say, Isaac b1837 and Katton were first cousins. It's a bit odd that some of Elizabeth and William's children retained the name Chew, as William Saywell was their acknowledged father.
Hence I say that the Bourn born Chews 1805-16 had no Chew blood in them (but Joseph bap 1799 at Southill also moved to Cambs, so his offspring born in Bourn are genuine Chews, but they all emigrated to Australia in 1854). Let me know if you need anything on the Rogers from Bourn. I can send you a full report on this family going back to 1655 if you send me a personal message with your email address
David
PS The earliest Chew reference in Southill was the marriage of Joseph to Sarah Reynard/Reynolds on 14 Feb 1720/1. This family may have been non-conformist as "sister Chew" became a member of Southill Baptist Church on the 1st of the second month 1725, and I can find no baptisms in the established church although there were marriages in the 1750s (my genealogical research is blessed with baptists in Southill, who didn't baptise their children!)