Hi Trish
Well this is a puzzle but how does this seem?
Samuel Chapman married Charlotte Gibbons June 1/4 1849 Holborn
Daughter Charlotte Ann was born June 1/4 1850 Clerkenwell
All 3 are on the 1851 census
Daughter Charlotte Ann died Sept 1/4 1851 Clerkenwell
Son James was born Sept 1/4 1851 Clerkenwell
Wife Charlotte died March 1/4 1852 Clerkenwell
Samuel (possibly) died at Kelshall March 1/4 1852 death registered at Royston 3a 128 (which is the reg district for Kelshall) after taking his son back to Hertfordshire to be brought up by family after the death of his wife.
No deaths for Samuel Chapman in Clerkenwell 1851-1861
(Possibly orphaned) James Gibbons Chapman grandson is on 1861 census with his grandparents.
It may be worth you posting on the Hertfordshire board to see if anyone has access to the Kelshall burial registers to see if Samuel is buried there.
If you got wife Charlotte's death cert you would find out possibly if her cause of death was infectious.
Then, if you're feeling flush, order the Samuel Chapman death cert Royston Mar 1852 just to rule him in or out if no-one turns up a burial in the registers.
Other than than, Samuel is still alive in 1861, his name and place of birth is completely mistranscribed or he's in one of the Islington missing pages. He doesn't turn up in 1871 either.
Dawn