I've now had my tensioner replaced, about 10 days ago.
There is not much space to get the old bolts out or new ones back in so I decided to let my local indie do the job.
They did quite a lot of removal to get at the bolts but at least that allowed them to get a torque wrench on the new ones.
In detail, 3 hours labour to remove then re-fit the following :
Air Box, Rear Lamps, Rear Bumper Skin, Front (transverse) Muffler and Muffler Support.
Support weight of Engine then remove the Engine Carrier which is the component in the way.
On re-assembly, new Cat to Muffler bolts (6) fitted.
Having asked for the old parts to be given back, I've cleaned them up to see what had failed.
The new Tensioner in place:
The bigger surprise was that it seems the Hydraulic Tensioner is seized solid, still full of oil. The Pivot Bolt for the Lever had become slightly loose which transferred all the movement in the system to between the Pivot Bolt and the inside face of the Pivot Sleeve which now has the start of a thread being formed inside. I guess the movement here could cause the Pivot Bolt to snap as the Tensioner mechanism was now solid.
Anyhoo, all's well again, for now.