I saw John and Isabella's firstborn given as Mary b. 1788 on an Ancestry family tree but have been unable to find her shown anywhere else so I have discounted her for the time being at least.

Following through with the William b. 1794 Braddan s/o John and Isabella (Cavendish) marrying Margaret Quine 1816 (cert obtained). I believe that John Gelling b. 1764 Braddan is the son of John Gelling b. 1740 married Mgt Quilliam etc..... as per my original posting part of which I have copied and pasted the relevant part as shown below. I obtained the will of John and Mgt Quilliam and although Mgt must have died prior to John as she is not mentioned in his will there is a John whose dob would fit in with when John and Mgt Qu. married in 1763. I agree with you that there were a lot of John Gelling's around at the time and is difficult to be sure we have the correct John G.

"John Gelling 1722 – 1812 M 1739 Catharine Killey 1712 – 1806
Children of JG and CK
John 1740- 1829 M 1763 Margaret Quilliam 1740 – 1822 Marown

Children of JG and MQ:
John 1764 Philip 1766 Margaret 1769"

I have made a note to obtain the wills of:

John Gelling d. 1732 marr. Margaret Kneale

John Gelling d. 1855 marr. Eliz. Cavendish

William Gelling d. 1871 marr. Margaret Quine

All shown on BL's DB, and which I hope will confirm my beliefs to date.