Agas dynergh Chris!
Trevellick is indeed a name found in the Isles of Scilly and although I don't have any connection I know of yet ( ;D) to this family, a quick look seems to suggest that the Scilly Trevellicks are one family.
There is also a Trevellick (place) in the Parish of Creed with Grampound.
I believe a variant of the name may be Trevellack- needs to be confirmed however.
If Trevellack is indeed a variant of the same name, i.e. Trevellick, then the earliest records seem to come from Gerrans. The placename in Creed-Grampound suggests that the name itself may not have originated in the Scillies.
Emanuel Trevellick dies aged 71 in 1822, buried June 12th- Scilly Isles.
Mary Trevellick dies in 1823- no age given, but possibly Emanuel's widow?
I had a rummage around the OPC and found the following for Trevellick in the Scillies.
groom 1838 Scilly, Isles of David TREVELLICK
marriages groom 02-Apr 1840 Scilly, Isles of (St. Marys) Richard TREVELLICK
marriages grooms father 02-Apr 1840 Scilly, Isles of (St. Marys) Emanuel TREVELLICK marriages brides father 18-Apr 1843 Scilly, Isles of (St. Marys) Stephen TREVELLICK
marriages bride 18-Apr 1843 Scilly, Isles of (St. Marys) Jane TREVELLICK
burials - 28-Nov 1849 Scilly, Isles of Emmanuel TREVELLICK
marriages grooms father 09-May 1854 Scilly, Isles of (St. Marys) Stephen TREVELLICK marriages groom 09-May 1854 Scilly, Isles of (St. Marys) Patrick TREVELLICK
banns groom 1854 Scilly, Isles of Patrick TREVELLICK
baptisms mother 17-Jun 1855 Scilly, Isles of Elizabeth Trevellick
baptisms father 17-Jun 1855 Scilly, Isles of Patrick Trevellick
baptisms - 17-Jun 1855 Scilly, Isles of Elizabeth Trevellick
burials - 26-May 1855 Scilly, Isles of Francis TREVELLICK
burials - 19-Dec 1855 Scilly, Isles of Betsy Ann TREVELLICK
burials - 09-Apr 1855 Scilly, Isles of Ann TREVELLICK
marriages grooms father 07-Nov 1861 Scilly, Isles of (St. Marys) Samuel TREVELLICK
marriages groom 07-Nov 1861 Scilly, Isles of (St. Marys) William John TREVELLICK
marriages brides father 04-Oct 1862 Scilly, Isles of (St. Marys) Samuel TREVELLICK
marriages bride 04-Oct 1862 Scilly, Isles of (St. Marys) Nanny Johns TREVELLICK
banns groom 1866 Scilly, Isles of Patrick TREVELLICK
burials - 16-Feb 1870 Scilly, Isles of Samuel TREVELLICK
burials - 31-Oct 1870 Scilly, Isles of David TREVELLICK
burials - 07-Feb 1870 Scilly, Isles of Ford Attwell TREVELLICK
marriages grooms father 13-Jan 1875 Scilly, Isles of (St. Marys) Emanuel TREVELLICK
marriages groom 13-Jan 1875 Scilly, Isles of (St. Marys) Emmanuel TREVELLICK
marriages brides father 15-Dec 1875 Scilly, Isles of (St. Marys) Patrick TREVELLICK
banns bride 1875 Scilly, Isles of Mary TREVELLICK
marriages bride 15-Dec 1875 Scilly, Isles of (St. Marys) Elizabeth TREVELLICK
burials - 13-Apr 1876 Scilly, Isles of Stephen TREVELLICK
burials - 11-Aug 1880 Scilly, Isles of Emanuel TREVELLICK
Moving on to the LDS the earliest record I can find is
Patrick Tryvallick, son of Emanuel baptised 1 Sept 1730
I notice the spelling does vary a lot. Trevalec, Trevalac, Trevellyck and so on. This may confirm the name indeed being the same.
It might be easier, as other have said, if you can provide some names and dates in order to pin down whom is being sought!
Hope that helps... have a rummage through this information and see if there are any links!
M