We have a similar arrangement. My understaning is the lane is "owned" by one of our neighbours but we have right of access. However it is not a public right of way (or at least was not internded to be one) so others using it without permission would be trespassers.

My (hazy) awareness of land law is that the period of 12 years may be significant. If someone occupies land unchallenged for that amount of time it's pretty difficult to get it back off them. If people have access to a path for that time it becomes a right of way (hence those signs you see saying certain paths are not intended to be a right of way - guess it's meant to prottect the owner's right to block it/build on it). But this may be a gross oversimplification!

In short I think "land grabs" are quite common and most probably go unnoticed