Thanks a million, Kath and hiraeth, for your help and thoughts!
Hiraeth, the passenger list sites I was able to look at didn't have boats coming into San Francisco. Just ports on the east side. New York seems to have been the biggest, so I suppose it is the most likely! I don't know if it's possible to check US immigration records, but I will have a Google and see what I can find!!
Also, the outgoing UK passenger records may or may not be useful. 1890 onwards is too late for all my folks, except one couple. A kind Rootschatter checked for that couple on findmypast.com, but the information is so sketchy and vague that I can't determine if it is them or not!!

And there is no word that the records are going to go earlier than 1890.
Kath, thanks for checking the 1850 census. The fact that he is not there still leaves this person "in the frame" to be possibly mine! I'd be grateful if you'd have a look at the passenger lists - as I say, I didn't come up with anything. If he went, it would have been the 1850s, and his daughters were withdrawn from their school in 1855, so that is when poverty and an absent husband/father began to impact the family in England. I don't know how to check US death records but will have a Google!
MarieC