…but I used Navigate on Autopilot a couple of weeks ago, and it was bloody useless

I don't lnow whether it was a particular set of circumstances that confused it, but it kept suggesting lane changes which were pointless - i.e. I would've had to have changed lane back again because I would have been on a slower car in a matter of seconds, or it just failed to suggest ones that I needed to make to maintain my speed. Of course, if you actually change lanes yourself you knock it out of AP anyway (but cruise stays on).