The legal status of the Isles of Scilly as part of the Duchy of Cornwall, or even part of England, is apparently not as 'cast iron' as you might (or as the Duchy might like you to) think. The doubts go back centuries and anomalies lasted into the mid-20th Century.
Dr John Kirkhope has produced a commentary on the matter based upon evidence found in the National Archives. Its title is "This Miniature Nation". It can be downloaded here .
Older islanders will know all this but as late as the 1940's most islanders were not expected to pay any tax!
From "This Miniature Nation". .........
It is recorded in 1946 that the people on the Islands were: “The sort of people who won’t keep proper records and are a nuisance from the Tax Inspectorate view.”
In the same year Inland Revenue said:
“I do not think we should put forward legislation at the present time. We should get very little tax at present we cannot afford to put trained staff on the job of getting the assessment..”
Dr Kirkhope intends making "This Miniature Nation". available free via E Book on Amazon in the future.