Mid to late eighties book from a UK library.
It begins as society is approaching breakdown, a heredity highlands chieftain is visiting the Glencoe Massacre monument and also mentally working out a plan for the future. Whilst he is in Glencoe at the start of the story he meets a "Bonnie lassie" and tells her that "she has this day met The McGregor".
Society collapses (maybe nuclear war too but I'm not sure?) and he ends up on a Outer Hebrides island. Slowly he uses his position with other clansmen to gain real power and finally he's got the authority to order men hanged or outcast.
Other small scale survivor groups establish trading contracts with his group but they eventually have to swear fealty to him.
I think there's a battle with fleets of fishing vessels and shotguns as well.