They can adversely possess the land - the way to do this is to exclude others from it - boundary structures are the usual approach. the risk here is that the council will askk them to take them down. It is for fairly obvious reasons not possible just to claim other peoples land in any circumstances. You must find a basis for a claim in law. The above categories are the only ways you can do this. If you can fit the circumstances to one of those legal rights you have a case. You could as a final alternatie ask the council if they would grant a licence to your parents in return for a small monthly fee they would maintain it for the council. I should be very grateful if you would kindly take a moment to rate my service to you today. Your feedback is important to me. If there is anything else I can help with please reply back to me.