The information about the two sons of the Hon David Spencer which is given on
thepeerage.com website has been provided, according to the footnote, by Thomas Ellison, who is presumably the elder of those sons, Thomas Felix Cameron de Charrière Ellison-Spencer. The younger son's name is given on
thepeerage.com as Sebastian David
Oliver Spencer.
I checked the GRO births index, and found the birth of Thomas Felix Cameron Ellison registered in Kensington and Chelsea in Q4 2006. The mother's maiden surname is not shown against a birth entry on the GRO index unless one inputs it as one of the search criteria. However, I checked this birth entry on the Find My Past website, where the mother's maiden surname is automatically shown, eliminating the need for the researcher to have prior knowledge of it, and found her surname to have been Warner.
It seems likely to me that the younger son is the Sebastian David Isaac Ellison (note the different third christian name) whose birth was registered in Kensington and Chelsea in Q4 2007. His mother's maiden surname also appears on the GRO births index as Warner.
If these two birth entries are, indeed, for the sons of the Hon David Spencer, this calls into question why they were registered under the surname of Ellison, which is something to which I do not have the answer.