A quick update to this thread to say that after quite a while with a very reliable chain tensioner I have just had a mishap. In view of the project to go back on the road I have pt the mtb back together (with cable discs front and back). In order to get to the gearing for road use I changed the sprocket from 21 to 18. Tuesday evening I had not yet changed the chainring and lengthened the chain when I took the bike out to go to the monthly club meeting. On the way there the tensioner was a bit noisy, but I thought with the extra slack in the chain the pulley isn't lining up so well. On the return the noise stopped, followed immediately by a skipping chain. Inspection showed the pulley was missing. That means it unscrewed itself in under 3kms (from the moment I changed the sprocket). Scary that it should do it so quickly. I looked but didn't find the old pulley, nor its bolt.
I have now fixed in a replacement pulley. The problem can arise because the rotation of the pulley is anti-clockwise and obviously a suble misalignment was sufficient to create the drag that unscrewed the fixing screw. To help things the new pulley has had its bolt loctited with Loctite 271, the strong thread locker. What it really needs is a lh thread but no chance of organising that.
A little warning to others with this sort of tensioner, a bit of loctite on that bolt that holds the pulley may help to make it secure. Also pay attention to strange noises emanating from the back of the bike.