Thank you so much for your quick replies!
I certainly didn't expect anything so quickly - cheers for that.
I don't think it's the same Thomas Thompson and Isabella, though. My Thomas Thompson died in India in 1840 (of Black Fever) while serving with the 3rd Light Dragoons. His widow died at sea in the same year - also of Black Fever - while returning home with their son. Their son (James Turner Thompson) was born in India and, on arrival back in England, was given over to the care of his father's family in Preston.
Turner Thompson (witness to marriage of John and Prunella Thompson) certainly explains the unusual middle name of my g-g-grandfather. On some census records 'Prunella' is spelt incorrectly.
I'm thinking that, maybe, the Thompson family originated in Northumberland region.
I can't find any reference to Thomas Thompson (or Isabella) prior to 1839 when they registered James Turner Thompson and had him baptised at Meerut, Uttar Pradesh, India. Thomas is buried at Cawnpore/Kanpur Cemetery.
Once again, thanks for your efforts on my behalf.