There is nothing to stop you paying the court fee yourself and litigating yourself. If the other party uses solicitors and your claim is not successful, they can claim solicitors costs against you.
What you have been told is not strictly correct. You tell the other person that if they don’t regularise the situation, and make the access a reasonable width, you will apply to court for a court order to determine the matter. If they just ignore it or refuse, you have no option but to go to court.
If you simply went to court without warning them, you could get penalised with regard to costs even if you were successful in getting the court order.
If he is building over a right-of-way which is a shared access, he is restricting that right-of-way. The court would also look at whether the restriction is substantial or not. For example, a wheelie bin which obstructs a drive which is 24 foot wide is not substantial obstruction. However on a path which is 1.5 m wide it is substantial. Therefore it comes down to the facts and evidence as to whether this is a substantial obstruction. Notwithstanding the dimensions, the fact that it has come down in size by such a large amount, I’m of the opinion that the judge would find it to be a substantial obstruction.
If he has not started building, it depends whether he also wants to risk many thousands of pounds just for the tiny amount of extra building.