Really? Whilst noone would talk ever talk or gossip about her engagements, I'd like to think that most people are at least aware of her existence; particularly given her competing in the Olympics and huge visibility as Britain's IOC representative and huge visibility in the run-up to this year's games.
Go out onto the streets of britain and ask people who princess anne is. I seriously doubt you'll get a good rate of correct answers, especially if you ask people under 50.
There are so many minor royals, and their lives form such a complex soap opera, it's very easy for them all to blend into one if you don't actually follow it, which very few people do.