Cut a hole in the inner wing to get at the belt tensioner?

[cheapo]

I have to admit I was puzzled by your suggestion too as was reasonably convinced the area between the engine bay n wing is all structural.  It's a frustratingly tight space to work in but do-able.

Heh, I haven't even bought a hole saw yet, so I might consider a 60mm hole, or whatever will allow me to get the tools through. I'll almost certainly give the job to my mechanic if the hole isn't feasible.