California to England

[skurrie]

Just wondering if anyone can help me, before l go completely bald!

My 2Ggrandparents John Henry STEPHENS/STEVENS and Mary (Trevena) both born in Cornwall England went to California in 1862.

Sometime between Aug 1870 and Sep 1877 they returned to England.

We have the shipping records for them sailing from Plymouth England to Sydney Australia  2nd sep 1877.
The family appears on the 1870 census in US, so they would have made their return trip between 1870 and 1877.
July 29, 1870 Federal Census of Greenville (Indian Township), Plumas County, California.
John Henry Stevens - 32 (born c. 1837/38 at England) -
Mary Stevens - 35 (born c. 1834/35 at England) -
William Henry Stevens - 5 (born c. 1864 at California)
James Stevens - 4 (born c. 1865 at California)
Mary Stevens - 2 (born c. 1867 at California)
Susan Stevens - 10 mos

Where do l start looking for what ship they may have returned to England on? l have left a couple of queries on US sites for any departure information.
Looking at many sites, you really have to know what ship to look for.

l'd be most grateful for any assistance and thank you in advance.

Skurrie