Hi Peter,

I always have to remember that there were two James Mansfield Gelling's. Yours was baptized plain "James Gelling" 1 Feb 1836 St. Peter Liverpool son of James Gelling & Martha Robinson, and the other "James Mansfield Gelling" was born in the Sept quarter 1840 Liverpool (no IGI baptism) son of William Charles Gelling & Winifred Poland. Turns out they are first cousins with grandparents William Gelling & Elizabeth Mansfield, as you have said. The marriage I have for them is 8 Aug 1803 St. Anne, Richmond, Liverpool. It's in Findmypast.
William Gelling, Cordwainer Liverpool
Elizabeth Mansfield, Spinster, Liverpool
Banns St. Anne's Richmond

Both William & Elizabeth Gelling are in the 1841 & 1851 census.

1841 census at Gellen Court, Liverpool HO107/556/16 20 pg 19 (306938)
William GELLING 60 ShoeM Y
Elizabeth GELLING 55 Y

1851 census at Tempest Hey side street (3) Liverpool HO107/2179 906 pg 9 #22
William GELLING, Head Marrd 72 Shoemaker, b Isle of Man
Eliz'h GELLING Wife Mar 66 Wife, b Lancashire Liverpool
[immediately below son James]

I don't know for sure, but the William Gelling bapt 11 Oct 1778 son of Charles Gelling & Elizabeth Quilliam (md 11 May 1770 Braddan) is a good candidate to match the one who married Elizabeth Mansfield. Charles' 1805 Braddan will is transcribed on the IOM will index. There were 5 children: John, Robert, William, James & Ruth Gelling co-admin's, and it says Wm is off the island. It also says "Letter from Liverpool 1 Jun 1811 from William Gelling received from hands of Robert Quilliam 2 pounds as right from fathr Charles Gelling". Charles' wife Elizabeth died a month after her husband and she also left a will but I haven't seen it.

Another possibility is the William bapt 1 Feb 1778 Braddan son of William Gelling & Catharine Stevenson (md 12 Jan 1764 Braddan). Wm was the youngest of 7 and his father died the following year. Mother Catharine md Thomas Cowley in 1785 and 3 more born. She died 1813 with a will that I haven't seen.

These two Williams were cousins, with grandparents William Gelling & Catharine Craine (md 28 Nov 1722 Braddan). Son William bapt 24 Feb 1729/30 and son Charles 25 Nov 1736 Braddan.

Another possibility is the William bapt 11 Feb 1775 Braddan son of John Gelling & Margaret Caroon, though he should be 76 (not 72) on the 1851 census if it's him. William received 5 pounds in father John's 1822 will. This family links into the "Schoolteacher Robert Gelling" clan that's been discussed in this BB before.

Through all of this, I can't see the connection to Jean Corkett's & Kay's Clegg's families, and to "relative Bruce Gelling in South Africa" who is connected to Jean & Kay. Could you fill me in? Maybe I've missed something along the way.

Since we're talking about William Gelling born about 1778-9 and Elizabeth Mansfield, I'm having some trouble with their children.
First one is James 27 May 1805 Liverpool and he's no problem, other than he added "Mansfield" to his name later.
Second is Jane 11 Aug 1808 but I have nothing more.
Then there's a big gap to William Charles Gelling bapt 8 Dec 1816. He married Winifred Poland 1839 and I've found 4 children. He's in 1851 and deceased by 1861.
Next is Thomas Mansfield Gelling bapt 23 Jun 1819 St. Peter Liverpool. Nothing more known.
Then another big gap to Mansfield Gelling bapt 25 Apr 1827. He married Mary Ward 1848, 2 children, and his wife married Henry Lyon just before the 1871 census. He apparently was a Mariner but isn't in any of the census's, nor a death record that I can find.
I also have a tentative Thomas bp 28 May 1826 St. Peter Liverpool to William Gelling & Betsy. Nothing more found for him and I'm not sure he belongs to this family, though I can't find anybody else so far.
Any clues?