A difficulty here is that, presumably for reasons of space, the Burke's Peerage article on the Grenfell family concentrates on the descendants of Pascoe Grenfell (1761-1838), who of course include the extinct Barons Desborough and St Just as well as the extant Barons Grenfell, but this Pascoe Grenfell's brothers and uncles are not named in the article, and their descendants are not shown there. However, it would appear that a relationship does exist in this instance.
When Julian Pascoe Grenfell married Diana M E Lane (registered Q1 1965 - Chelsea), his christian names were shown in two formats in the GRO index - "Julian P" and "Pascoe J". Similarly, when he married Sylvia R Wood (registered Q1 1988 - Surrey NW), his christian names were again shown as "Pascoe J". This led me to believe that his first given name might have been Pascoe, rather than Julian, despite the later ordering. It turned out that his christian name at birth had, indeed, been registered as Pascoe (Q3 1936 - Chelsea), and his mother's maiden name was shown as Plowes.
Pascoe E Grenfell married Joan Plowes (registered Q3 1929 - Kensington). As well as Julian Pascoe, they were also parents to a daughter, Barbara (birth registered Q4 1939 - Amersham) - presumably, she is the "Bobby" mentioned in her brother's death notice.
Pascoe Ernest Grenfell was born on 6 April 1904 (registered Q2 - Reading). His wife, Joan, was born on 31 October 1905 in South Africa - there is a family tree for her on geni.com at https://www.geni.com/family-tree/canvas/6000000060838046936. It appears that they emigrated, with their two children, to South Africa in 1946, although Julian obviously returned to the UK at a later date.
Pascoe Ernest Grenfell is shown on family trees posted on the ancestry.com website as either the son of Charles Spurrier Grenfell (1877-1909) or of his brother, Arthur Gordon Grenfell (1875-1923); his mother's name is in each case shown as Winifred Haskins. Family trees on the ancestry website need to be treated with caution, as some of the relationships shown there are speculative, to say the least. However, the marriage certificate of Pascoe Ernest Grenfell shows his father to have been Arthur, and this appears to be conclusive. It is also borne out by a family tree on geni.com at https://www.geni.com/family-tree/canvas/6000000040599758951.
Arthur Gordon Grenfell married Winifred Haskins in Rhodesia in April 1903. His ancestry can be found on the Grenfell family website at http://www.grenfellhistory.co.uk/familytrees/grenfell-o/up/chart-001.htm#aui74-511http://www.grenfellhistory.co.uk/familytrees/grenfell-o/up/chart-001.htm#aui74-511. On this basis, the late Julian Pascoe Grenfell was therefore a fourth cousin once removed downwards of the present Lord Grenfell through their common descent from Pascoe Grenfell (1729-1810) and Mary Tremenheere.
A further point of interest is that Winifred Haskins' sister, Lillian Sealey Haskins, married as her third husband (she was his second wife) Lieutenant Colonel Sir Francis Claude Shelmerdine CIE, OBE, FRAeS (1881-1945), who was Director General of Civil Aviation in the Air Ministry from 1934 to 1941. Their first cousin, Minnie Louise Haskins (1875-1957) was a poet and academic, best known for being quoted by King George VI in his Royal Christmas Message of 1939. Her poem is inscribed at the entrance to the George VI Memorial Chapel in St George's Chapel, Windsor, and in a window at the Queen's Chapel of the Savoy in London. It was also read at the funeral of HM Queen Elizabeth The Queen Mother in 2002.