I fancy it is the legendary Jeannie Longo?
She was still racing in her fifties at international level.
Chris mentioned the great Beryl Burton.
When it comes to an argument about who is/was the greatest British cyclist ever, I think I would go for Beryl.
She hammered the men as well as the women!
I seem to remember she caught the great Mike MacNamara (think it was Mike) in a time trial, offered him a sweet, and said "See you at the finish, Mike", and hammered past him.