"Sadly, in the two recent YouTube clips I could find (July, at the Half Moon, Putney), the band seem a bit out of voice. Maybe this appearance was just after a big festival?"

The answer to that question is NO - that's how rubbish they were.

Who loads up a clip onto Youtube of a rubbish performance ? If you're uploading material surely it should be of your best performance...

Don't get me wrong. I do admire and respect John Tams - he's done some great stuff, written some great songs, he has a great voice.
But just because something is heartfelt and worthy doesn't excuse it. I don't care if it's "heavy folk" or "folk rock" or any other sub-genre you want to invent to try and excuse it - if it's shit, it's shit.

I sense a slight case of 'Emperors New Clothes' here - everybody seems too willing to say it was good just because it's John Tams - but I think you'll find it was neither folk nor rock, it was 25 years out of date, it didn't inspire or uplift or excite - if you'd wandered aimlessly into some pub and some average pub rock band were performing that, you'd ask for your money back. I can hear guitar solo's like that in any guitar shop on a Saturday afternoon.
I stand by my original criticism. Dreadful.