If the land is an adopted highway then it is presumably owned by the Council. In that case, if people are encroaching on the land the Council ought to be concerned and take action. If they don't (& your contact with the Councillor doesn't produce a result) then you can follow the Council's complaints procedure and - if still not satisfied - then go to the Local Government Ombudsman.
However, you really need to be sure of your facts. What is the actual width of the land the Council own? Could it be that it is narrower than the land which has previously been used and the neighbouring landowners are simply putting fences etc. up at the correct bounday of their land? Try to get a Land Registry plan, which might show the proper boundaries of the lane - go to the land register online site.
Something else you need to check is whether your own title deeds give you any specific right of way over this lane. If so, what are the details and who granted the right?