Similar thing happened to my neighbours on the next block to me. They ended up in court after loads of hassle.
They said that if it happened to them again they would try to avoid all the expense and get plans from the council showing each property access and let someone legal like a solicitor mediate and explain the access situation.
These people must have been made aware of this at the beginning of moving into the property because if new tennants are private purchasers the people who sold it to them and their legal people (even also any agents) would have definately been aware of the shared access, and must by law have informed them, and because it would have shown on the deeds).
Even council tennants are informed of this and the legal implications because the council do not want any hassle.
But if it just something that has not been put on the deeds and this is a loophole, you may I believe (but don't quote me) have legal rights after usage of more than 12 years, because the law says that this is reasonable.
Also If it is not on the deeds then there is something also called an 'easement', whereby usage of access points belonging to other people over long periods of time can be (but only by the court) given as right to the people who have usage over long periods with permission from previous owners. Unfortunately it then gets very legal.
I hope that this can give u some idea of what is what. But I would tread very lightly at first and get ur facts right and get plans from the council to see exactly where u stand before saying anything more.
Regards Bluebottle, good luck.