Thank you for this prompt reply.
The UK Border Agency's website seems to say that, as the wife of a British citizen, only 3 year's residency is required.
"Can I be naturalised as a British citizen?"
"If you are over 18 and have been living in the United Kingdom for the last five years (or three years if you are married to or a civil partner of a British citizen) you may be able to apply for naturalisation as a British citizen"
So is it three years or five years? And what are the allowable periods of absence abroad for the three year category?
I am afraid I forgot to mention a relevant fact in my original email to you. I knew my grandmother's place of birth was relevant but I could not remember why.
On 2 May 1989 (10 months before I got married), I went to the Home Office Immigration Department (as it then was) in Croydon, taking the birth certificates of my grandmother (the one who was born in London) and my mother. They gave me on the spot a stamp in my passport: "Given leave to remain in the United Kingdom for an indefinite period". I believe "patriality" was the term they used. I still have this old passport. So my indefinite leave predated my marriage.
Does this change the residency requirements?
Kindly rate my answer positively so I can get credited for my time.