I'm grateful for the very helpful replies.
I had the information about James and his siblings from the will of his relative Mary Tomkinson:
Extract from will of Mary Tomkinson of Dilhorne 9 July 1810:
'The Real Estate left to her by her late brother John Tomkinson and all other Real Estate whatsoever to my relations James Tomkinson, a Captain in the navy, Henry Tomkinson of Birmingham, cooper and Thomas Tomkinson of (left blank) surgeon and apothecary, sons of John Tomkinson who died at Shardlow, their heirs and assigns as tenants in common and not joint tenants.'
The age of 56 at death is very helpful. The nearest baptism I can find is of James Tompkinson, baptised 13 Feb 1782 at Ellenhall, the son of John and Elizabeth. This could be him as Ellenhall is near Knightley, and the date could be consistent with an age of 56 at Death.
I have been unable to find a marriage record for his parents. There is a record of the marriage of John Tonks to Elizabeth Bailey in Castle Church, Stafford, 31 Dec 1780/1 Jan 1781, which I'm wondering could be this. Castle Church is quite near to Knightley, and other Gnossal Tomkinsons married Baileys.
Don