Success
- managed to take the bolt off. A 6-point wrench was still starting to slip but I got this anti-slip wrench (Jonnesway W301 - see photo in attachment) - which hold the bolt tightly - I highly recommend this tool if anyone is having problems removing any bolt.
Looks like whoever was taking this tensioner apart before - lost the washer and then just tightened the crap out of this bolt to prevent a leak.
Anyway - the old tensioner looked like had very little wear (see photo with old and new in extended position - and a mangled bolt) and was not expecting it would solve my rattle problem. I've put the new tensioner in with a new washer and torqued it to 40 Nm - started the car, it rattled for a while when the part was filling with oil and then the rattle stopped. I drove the car for a couple of days to check it under different conditions - the rattle is gone!
