OK...so armed with mirror (thanks to the missus), torch and Mk1 eyeball, I popped out to the car, located the bung and had a gander.
It would appear that the tensioner piston is just about fully compressed and has hardly moved since being re-set at the cambelt service 24k miles ago. I think I can see that the locking holes are just about still aligned.
Question...is it possible to install a replacement tensioner wheel so that when bolted to the block it is pinched tight, therefore cannot rotate and will simply keep the piston compressed? Or is it impossible to do that with the design of the bolt / attachment? Just thinking of scenarios.
UPDATE: I think Gary (Lovel) has experienced the binding of a non-branded tensioner owing to the thrust washer / spacer tube sizing. See HERE
Otherwise it looks like the piston is not doing its job of taking up the slack on the belt. From the surface appearance (i.e. general / light reddish brown surface rust I think the hydraulic tensioner is the original and wasn't replaced at service.
If I am correct, then that means there is nothing to chase on parts/warranty as the usual recommendation is 100k miles for the tensioner...but as Gary said, the unit can simply be weak / failing and not doing the job.
I will snap some pictures of what the mirror reflection yielded in case it is useful to everyone. The bung obviously allows inspection of the tensioner position so that assessment can be made of what is happening.
Onward with the diagnosis!
Jonty