Thanks everyone.
I too found Fanny as a servant girl in 1871 (I only found her once though - lost it afterwards, never to be found again!). You are of course right about the marriage - it was in Kensington, not Kent. Their son was born in Kent, and I was working from memory. Sorry for the confusion!
I'll work through the messages and see whether anything fits in with any other information I have. It hadn't occurred to me that Isaac could have been a fictitious father's name. That's of course possible. Without knowing the correct birthdate, I can't get the birth certificate which would confirm father's name - and don't know her mother's name! If she wasn't a servant in 1871, she might have been at home with her parents! Which is a shame!
JB