Note: since writing this, the webshite now runs on WordPress which has commenting built in. Nobody used the IntenseDebate system anyway!
I’ve finally pulled my finger out and got a comment section set up on trial. Just click onto a post’s own page (if you’re on the home listing) then look for the ‘Comments (#)’ link at the bottom of each page to pop up the comment box. The system is powered by Automattic’s IntenseDebate which is delightfully old school but appears to do what I need it to in a competent manner, in contrast to the half-baked home-spun Grav-specific offerings.
No login is required to post, but there is an option to login with an IntenseDebate or a WordPress account should you prefer. Posts won’t appear straight away as I’m initially moderating all comments to ascertain what the spam bot level is like.