Thanks for the feedback and pointers, I have been plugging away at this and think I have a story that fits, but proving it is still nagging me!
In Oct/Dec quarter 1862, John Collins married Eliza Ann Woodland at Wareham, Dorset.
In the 1871 census John (33, masons labourer) and Eliza Collins (33) are living in Dorchester, with sons William (7) and Henry (2). John, William and Henry are all born in Wareham.
In the Jul/Sep quarter of 1871, a Henry Collins (2) died at Dorchester.
On 28 March 1876, Robert Charles Collings was born in Clifton, Bristol, to parents; John Collings (excavator) and Eliza Ann Woodland. The father, John, is the informant on the birth certificate and the birth was not registered until 4 May 1876.
An Elizabeth Collins (37) died in the Apr/Jun quarter of 1876 at Clifton, Bristol.
In the 1881 census a John Collings (46, road labourer) born in Dorset is living as a boarder in Clifton, Bristol.
In the 1881 census William Collins (17, born Wareham, Dorset) is on board HMS Cambridge at Plymouth.
Robert Charles Collings has not been traced in either the 1881 or 1891 census.
Robert Collings married Alice Elizabeth King on 5 June 1897. The certificate records his father John Collings (labourer) as deceased.
I suppose it is the interchanging of Collins and Collings that still nags at me, and not being able to find the wherebouts of Robert in either 1881 or 1891.
Robert was born at 3 Cates Cottages, Clifton, Bristol. Can anyone suggest where to look for his baptism - this may confirm whether his mother was still alive.
Would the death certificate give any useful information? Presumably it would perhaps clarify if Elizabeth Collins died as a result of childbirth.
To me the balance of probabilities appears to point to this as the same family, but am I just making a story 'fit' the information I have. Any suggestions as to ways of verifying the relationships would be appreciated or am I just getting picky now?

David