Yes, Geoff, you are right, I am presuming too much.
I found Henry, Margaret, Martha Jnr, and Thomas (together with their parents) in the 1861 census with the surname of Bell, at a farm called Hacen Bags, Petteril Crooks, and have assumed they were 'always' called Bell.
Maybe Robert Bell took pity on Martha and 'took in' all her kids when they married and he was only actually the natural father of William.born in 1862?
I think I will try the Parish Registers at Carlisle to see if any father is mentioned for the early children of Martha, and then maybe try some birth certs., but make sure there is a father mentioned before I buy.
I feel that you have at least put me on the right track.
Once again, many thanks.
Gerry