Hi anglecreek. I had a look for John with a bro. Richard in Hartland, Devon in 1851 and 1861 census. No Joy.
Searched same in just Devon and found a 17 year old John, born in Littleham, 15 miles foom Hartland, but working as an apprentice to a tailor and inn keeper in Weare Giffard a few miles furthe away from Hartland.
Other 7 persons are all south devon based. 50 miles away.

Immigration records don't start soon enough for them. They may be on an arrival register somewhere in Oz? 4 shown are all born after 1893.
PS it is a lovely dry warm day here in North Devon. Regards PeeDeeBee

Richard according to the bio I've found, arrived on the George Marshall. It only made 2 voyages to Australia one in 1854 and the next in 1855 so must be this one:-
17 December 1854 sailed from London for Port Phillip (Capt Godfrey)
21 Dec arrived in Plymouth 21 December then sailed for Melbourne on 27 December.
(someone has something wrong).
MARSHALL, RICHARD (1838-1923), farmer and wheat-breeder, was born in 1838 at Hartland, Devonshire, England, son of Richard Marshall, and was educated at local schools and Shebbear College. Young Richard reached Melbourne in the George Marshall at the age of 16, and worked in a market garden at Geelong. In 1854 he joined his brother John at Yankallila, South Australia, and later farmed in a partnership with John Way at Second Valley. On 13 September 1860 at the Bible Christian Chapel, Bald Hills, Yankallila, Marshall married Mary Ann Watson. They were to have ten children. Marshall farmed at Rapid Bay, but disposed of the property to go to the diggings at Sandhurst (Bendigo), Victoria. After two and a half years, he returned to farm for three years at Lovely Valley, near Myponga, where he suffered successive crop failures. In 1865 he moved to Wasleys, renting 246 acres (99.6 ha), but lost his first crop there to red rust (Puccinia graminis), and suffered further losses in 1866-67. However, he successfully established his property, Hope Farm, ultimately expanding it to 3800 acres (1537.8 ha). His farming encompassed cereal growing, dairying and sheep raising.

After his losses from rust, Marshall began to improve disease resistance, selecting plants from available wheat varieties in 1868, notably Purple Straw. In 1880 Richard Schomburgk imported Du Toit's (otherwise known as Early Baart) from South Africa. From this, James Ward, a farmer of Nelshaby, near Port Pirie, selected Ward's Prolific. Marshall likewise experimented with wheats he imported from France, Germany, England, Russia and the United States of America: in July 1892 he had 180 varieties growing in plots at Hope Farm, including 100 new ones. As well as rust resistance, he sought other selection characteristics, including likelihood of grain shake-out when ripe, quality and yield of grain, stooling, and strength and length of straw.

The Intercolonial Rust in Wheat Conference in 1896 recommended a change from the still widely grown Purple Straw. By 1910 this was almost totally replaced by Marshall's No. 3, probably a natural cross between Ward's Prolific and Purple Straw. Marshall's variously selected and cross-bred varieties?such as The Majestic, Gallant, Silver King (a white-grained selection from Marshall's No. 3), Phyllis' Marvel, and Dart's Improver?were distributed among farmers about the same time as those of W. J. Farrer. Marshall's most successful variety, Yandilla King, which was widely available from 1907, was derived from a cross between his Silver King and Farrer's Yandilla and was a 'half-sister' to Farrer's well-known Federation. Until the 1920s, Marshall's No. 3 and Yandilla King remained standards of yield among later-maturing varieties throughout Australia.

Marshall was a member of the District Council of Mudla Wirra North for seven years (chairman 1876-80) and a long-serving committee member of the Gawler Agricultural Society. He retired from active farming in 1899 to his Adelaide residence, Hartland, 73 Frederick Street, Unley. Two of his sons continued to farm at Wasleys, while the other two established farms at Lameroo and in New South Wales at Cootamundra. In 1901 Marshall was appointed a member of the Central Agricultural Bureau, continuing with its successor, the South Australian Council of Agriculture until 1905. In 1904 he received an illuminated address from the Agricultural Bureau of South Australia in recognition of his work.

Marshall's first wife died in 1905. On 3 July 1906 at Southwark, Adelaide, he married Elizabeth Richards. In the last years of his life he was blind. He died at home on 5 August 1923 and was buried in West Terrace cemetery, leaving an estate sworn for probate at ?29,700. His wife, and four sons and five daughters of his first marriage survived him. Although his work has been overshadowed by that of Farrer, Marshall left 'an indelible record on the scroll of Australian wheat breeders'.

The sons of Richard Marshall and Margaret, nee Fry.
Richard Marshall b: 1838, Hartland, Devon. died in Unley, South Australia on 5 August 1923. He married Mary Ann Watson 1838-1905 on 13 September 1860 at Yankalilla, South Australia. They had 10 children:- Mary Margaret, Frances Jane, 1863. Matilda Elizabeth, 1864. Henriette Estella, 1866. Frederick George, 1868. Grace Alice, 1870.
Albert Frederick, 1872. Walter Richard Fry, 1872. Oliver George 1876.
Hercules Charles Percy Harris 1878.
John Marshall was b: 1832 and died at Koorawatha New South Wales in 1904. He married Marie Ann Norman 1834-1871 at Hindmarsh in South Australia on 1 December 1854 together they had 11 children:-
Thomas Fry,1856. Thirza Jane, 1858. William James, 1859.
Dorcas Elizabeth, 1861. Frances Maria, 1863. Ellen Norman, 1863. Rosa Augusta, 1865. Clara Evangeline, 1868. John Robert, 1869. Grace Edith 1870. Maria Norman, 1871. After Marie died he married Priscilla Abbott 1840-1933 and had 2 more children:-
redrick Laurence George 1874 ? 1913 and Edgar Percival Stanley, 1876 ? 1936

The bottom bit starting from [The sons of Richard Marshall and Margaret, nee Fry. ] is by me. I'm not related just gathered it from a quick look around. So check it!