Finally got round to doing this today. I can confirm you can get to the belt tensioner fairly easily from above the engine so there maybe no need to jack the car up and remove the under tray. I don't have the biggest arms and I also have the Mishimoto fan shroud which makes it easier. I could only get maybe 1/2 to a full turn onto each tensioner before it was too tight to move. Doing it from the top doesn't allow much room to generate torque on the wrench.
Could someone confirm that tightening the screw (i.e. clockwise), tightens the belt (i.e. moves the pulley up). I tried to eyeball it but couldn't actually see any movement.
I also put some climbing chalk on the underside of the belt, I just coated my fingers in it and brushed them onto the underside of the belt, turned the engine over to spin the belts then repeated the process.
As the issue was intermittent I'll give it a week and report back.