OK, I'm intrigued enough to take it a bit further.
It sounds like the local authority may have made an assumption about the width of the verge that may have extended it beyond what is reasonable in common law. How wide is this verge to the road-side edge of your ditch? How far beyond that is this box that mobile company Orange has put there?
Yet more - what were you hoping for as a resolution? - removal of the box or payment of rent for the use of your land (if proven), or something else?
The council clearly aren't going to do anything unless you stump up the big sum of money they want. They hope that will dissuade most folks from pursuing it. Are you going to be prepared to start writing letters to them challenging this? - I can perhaps point you in the general direction but I'm not going to draft letters!
Meanwhile have a look at this, which explains how messy these situations can become (scroll down to the bit about boundaries with roads).
http://www.boundary-problems.co.uk/boundary-problems/titleplans.html