There is definitely a baptism record in the Crowan Register and the Bishops Transcripts for Dorcas Bosanko in 1721 and the father's name in this record is definitely Reginald. You can pencil that in as documented fact.
The fact that someone has Thomas as the father may or may not have some evidence to support it and at the moment a Will would seem the most likely source.
Aha! - I've checked Ancestry Public Trees and located one where Thomas Bosanko is listed as the father of Dorcas. A date of 20th April 1723 at Crowan is recorded so I've checked some more and found the details.
The page of the Crowan register on which this event was recorded is torn so the name of the child is missing as is the actual date - but there was a child baptised at Crowan in April 1723 to Thomas Bosanko.
I've also checked the Bawden Transcript which shows a reference in the 1723 Bishops Transcript which records that Dorcas daughter of Thomas Bosanko was baptised at Crowan 20th April 1723. Unfortunately I cannot find that particular Bishops Transcript in my collection so cannot check it but it does seem that the information is probably correct!
So your task now would be to try and sort out which is correct and I think the best way to do that is to establish if Reginald and/or Thomas Bosanko left a Will.
Okay, to save you a bit of work I have now checked for Bosanko Wills at Crowan. The first one I found was for Reynald Bosankoe of Crowan, tailor, dated 1760. This Will is quite clear and comprehensive but does not mention a daughter Dorcas or any son-in-law or grandchildren named Jewell.
I then looked for a possible Will for Thomas Bosanko and found one dated 1784 which was the only one that might be of some help. I think this one gives us the clue!
The first bequest is as follows:-
I give to Darcus Juel the sum of to shillings and SixpenceThomas does not specify any children but those named are inferred to be sons and daughters. Those named are:--
1. Darcus Juel
2. Samuel Bosanko
3. Florance Bosanko and her son Thomas
4. John Bosanko of Wendron
5. Thomas bosanko named executor so presumed to be eldest son
This would seem to indicate that it was indeed Thomas Bosanko who was the father of your Dorcas who married William Jewell.
The CRO reference for the Will of Thomas Bosankoe is AP/B/4596.
I've sent you a copy of the Will of Thomas Bosanko via email.
CT