Well spotted Steve - I've had my head in wills and OPRs and Census info. It looks as if the Auckland Colvin was the son of a brother of Mary/Maria but the Manchester info is also interesting.
I could find no will for James Colvin on SP. I've not tried the Nat Archives.
It might be worth Nessie searching for a death in England. However, it might well have been in India.
I know evey little about the Scots in India, so maybe specialist Indian sources/experts is the way for Nessie to go.
Gadget