OK, for the benefit of anyone considering new shocks for the XC90, I'll answer my own question
I did the job yesterday in under an hour.
Rear wheels off, axle stands under the subframe.
The only difficulty is holding the TORX screw at the top of the shock while undoing the 18 mm nut at the top mounting.
Top mounting is under the 3 row seat cushion, pull back the carpet cut the insulation with a craft knife (it's obvious where to make the cut) and remove the two dust covers.
Volvo have a special socket for this, but if you clamp an 18 socket really tight in a vice grip you can get at the torx through the middle of the socket and hold it while releasing the top nut
After this its just a matter of releasing the bottom 17mm bolt and pulling it out the bottom. You'll have to jack up the suspension to get it to clear the spring on the way out.
As they say, installation is the reverse of removal
I think the "tensioner" must be to keep everything at ride height while changing suspension components, probably more important for bushings and the like.
When it's all back together, the measurement between the top of the wheel arch and the center of the wheel should be 453mm
Jobs a good un'
Mark.