There are at least two more records I might buy later.
https://www.oldscottish.com/seafield.html
On this page John Christie Richardson born 18th December 1874.
He is on the 1881 Census with his mother Elizabeth and grandmother Jane Richardson as John C Dempster. Elizabeth was my 2x-great-grandmother Isabella's sister.
He later took the surname McBeath when his mother Elizabeth married William McBeath.
Secondly, the record I found first. My 2x-great-grandmother had another illegitimate daughter in 1870 - Anne or Annie.
https://www.oldscottish.com/grange.html
I clicked to order this but also put a query about my great-grandmother Isabella, and they replied to that but not Anne. I suspect her father's surname was Cameron as she was given as Annie R Cameron on one census.