Thanks everyone, for the suggestions for "Colpton", and for the 1851 census
details. You've been extremely helpful.
The consensus seems to be that "Colpton" was an abbreviation for
"Cullompton", which I hadn't heard of, but which is apparently near Tiverton
and even has its own website at www.cullompton.org
There were GILLARDs resident there in
1851 (although no William), and others resident nearby at "Cove Down, Tiverton"
(including a William of the right age, i.e. born 1850).
Thanks again, and if anyone knows anything further about GILLARDs in the Tiverton /
Columpton area (or in Colyton) then do please let me know...