I think 'arran sweaters' is a polite way of saying 'those grumpy ol' buggers who moan at anyone who's not part of their club'

Yes, you're right, Lizzie. It's become a shorthand term for the old stereotypical traddie, just as 'anorak' is shorthand for an obsessive person, immersed in trivial details about their hobby or interest (e.g. philately, trainspotting, or folk music - ooerr!) :-)