Hi
Leonard and Harriet (who was with Leonard's son George on the 1841 census in Newington) are the children of Leonard Cubitt and Mary Smith (possibe marriage according to somone on the internet in 1784 but I have no eidence of that). Leonard junior's first marriage to Mary Ann Cubitt is probably, but needs to be proved, a first cousin marriage to the sister of Thomas. Sophia's signature at that marriage may be as the sister of the bride but could equally be as the sister of the groom.
If Sophia was the sister of Thomas then George Barrett secondly married his first wife's niece. If Sophia was the sister of Leonard and he was a first cousin of Thomas and married to his sister Mary Ann, then Susanna's relationship to Sophia was first cousin once removed.
The use of the surname Moseley as a middle name occurs in a child of Thomas', a child of Susanna's, Thomas' daughter and a child of Mary Ann, who looks to be Thomas' sister and Leonard's first wife. The surname does not seem to be used with any of Sophia's children or children of Leonard's second marriage. That doesn't prove Sophia and Leonard were brother and sister or indeed first cousins by a further as yet unknown Cubitt brother that Sophia could be the daughter of, but it does give circumstantial evidence for Sophia not being a sister of Thomas. Her potential birthdate would equally fit for a child of the marriage of Leonard senior to Mary Smith.
The John Cubitt and Mary Moseley marriage is on Family Search.
Charles Moseley Barrett would be named for his great grandmother not his grandmother (his maternal grandfather Thomas' mother).
Someone has posted on the Internet back in 2002 that the Leonard Cubitt who he claims married Mary Smith in 1784, he doesn't say where, was baptised 10th April 1757 at St James Pockthorpe Norwich the son of Joseph and Mary. A Joseph and Mary Cubitt do have a son John baptised 4th December 1763 at St Peter Parmentergate Norwich shown on Family Search, but the surname Cubitt looks to be quite popular in Norfolk so there may very well be more than one Joseph and Mary Cubitt and Pockthorpe baptisms don't seem to be on Family Search. That church's full title appears to be Norwich St James with Pockthorpe.
https://www.familysearch.org/learn/wiki/en/Norwich_St_James_with_Pockthorpe,_NorfolkI think the Second Foot Guards are the Coldstream Guards. The Guards regiments still maintain their own records.
http://www.theguardsmuseum.com/Family-ResearchThomas' name does not seem to appear on the Waterloo medal roll. The Waterloo medal was the first medal issued to all ranks of soldiers, so it looks as if he missed out as far as the medal goes. Waterloo medals nowadays are not cheap to buy.
Regards
Valda