In IGI :-

William Corkill married Jenny Blake in Padstow 1771.

A William Corkill and a Jenny had the following children christened at St Nicholas Whitehaven:

William 1775
Dorathea 1780
Jenny 1777
Blake 1789

Also at St Nicholas the following children were christened to William Corkill and a Jane:-

John Blake 1773
Richard 1783
Merria ( Maria) 1783

The sequence of dates fit the same parents.

Back in Cornwall, John Blake Corkill married Melicent Avery in 1796 and a John Avery Corkill married Susan Nankwell in 1824.
The use of the mothers maiden name may be significant.

Could this add to your Whitehaven- Cornwall theory.

Len Faragher