Hi
If in the meantime you have found what happened to David WALKER, b 1821 in Ellingstring, Blacksmith, who was sentenced to 7 years transportation in 1851 at Northallerton, then I hope that my public reply might help some others looking for answers.
I am as certain as I can be that the following is what happened to him. He wasnt transported, but ended his days in Dartmoor Prison: 24th Feb 1854.
I have followed what happened to him in the prison records, obtained from a commercial family history site, and the records essentially all confirm that it is this David Walker (confirmed by his trial date and place and occupation and age). I am descended from his half-brother William Vayro. Thus we share a direct ancestor: Mary Stott (my 3X grand-mother).
He was in Northallerton prison till 3rd Oct 1851 when he was transferred to Millbank Prison. On 8th Nov 1851 he was transferred to Pentonville. He was there till 2nd Aug 1852. The record says he was transferred to Portsmouth, but I dont have any record of him there. However shortly thereafter he appears in the Dartmoor Prison records from the quarter ending Sept 1852 and through to his death on 24th Feb 1854. Whilst in Northallerton it says he was suffering from scrofula (tuberculosis of the neck lymph glands, often caught by drinking infected milk), and in fact was first rejected by the surgeon for transfer to Pentonville. In the Dartmoor records he is described as scrofulous, and latterly "delicate". My guess is therefore that his death registration record is: Name: David Walker, Registration Year: 1854, Registration Quarter: Apr-May-Jun, Registration district: Tavistock, County: Devonshire, Volume: 5b, Page: 213.
I hope this helps.
Liz