I can help with the Gelling ancestry, and have Bridson back one or two generations. I haven't researched downwards beyond the 1899 marriage, so not much help with Heather's main query.

I'll include here the census entries for Paul & Sarah. In 1901 they are a couple with no children. In 1911 he probably is the seaman aboard the "King Alfred" in Torquay, while his wife and 4 living children are in Douglas. Her entry says that she has had 8 living children as of that date, 4 of whom are dead. Presumably the missing son James comes after the census date April 2 1911.

Paul Bridson was baptized 12 Apr 1879 Douglas, only son of Paul Bridson and Elizabeth Margaret Gelling who married 12 Oct 1865 Braddan. He had 4 sisters. Paul Bridson Sr was baptized 28 Oct 1836 St Barnabas Douglas s/o William Bridson and Mary Hogg, and died after 1901. Elizabeth Margaret Gelling was baptized 7 Apr 1839 Lonan d/o Robert Gelling and Eleanor Kinley and also died after 1901. Paul Sr was also a Seaman.

1901 census at Muckles Gate, Douglas RG13/5305 136 pg 5 #28
Paul BRIDSON, Head M 21 Mariner, b Isle of Man Douglas, English
Sarah BRIDSON, Wife M 19, b England, English

1911 census at 5 Athol Square, Shaws Brow, Douglas RG14PN34661 RD635 SD3 ED18/8 SN110 (2 rooms)
Sarah Eleanor BRIDSON, Head 29 Married 11 years, 8 born 4 living 4 dead, b Whitehaven, Manx [her nationality], English [ie, speaks only English, not Manx]
Mona Alexandra BRIDSON, Daughter 8, b Isle of Man Douglas, Manx, English
Lillian Bertha BRIDSON, Daughter 7, b Isle of Man Douglas, Manx, English
Paul BRIDSON, Son 5, b Isle of Man Douglas, Manx, English
Whorrall Henry BRIDSON, Son 3, b Isle of Man Douglas, Manx, English

This is probably him:
1911 census aboard the "King Alfred" at Torquay, Devon RG14PN12788 RD272 SD6 ED30 SN9999 Page 15 Line 10
Paul BRIDSON, 31 Married, Seaman Royal Naval Reserve, b Douglas Isle of Man

The Gelling line goes back the late 1600s in Ballamillaghyn, Braddan. John Gelling b 1705 was an illegitimate child of John Gelling (ca 1660-1733, md Susanna Moore ca 1685) & Ann Kneale (bd 27 Feb 1713/14 Braddan with will mentioning her son); he married Ann Fayle likely abt 1732 Marown. Their eldest son also named John married Margaret Caroon 1767 and son Thomas b 1769 married Elizabeth Kelly 1798. Their son Robert b 1806 married Eleanor Kinley 1836 and they are Heather's ancestors.

Please let me know if you'd like more detail.