Peter,
By "adjust the damper tension", I assume you mean the spring preload here. At the front, there is a 5 position preload adjuster that you need the C-spanner from the toolkit to adjust, and at the rear is the remote handwheel adjuster.
By increasing the preload (or tension), the bike will sink less for a given weight/load, so in a way the height is adjusted.
If you want the whole bike to sit lower when you are on it, you can wind preload completely off both front and rear. Surprisingly, this does not have any great effect on the overall handling of the bike (providing you're not 30 stone or more!).
If you want to change the rear ride height without affecting preload, adjustable paralever arms are available from the likes of Wunderlich / MV Verholen. See
Here for the MV Verholen version.
The rear damper also has compression damping adjustment (screwdriver required), but this will not affect the ride height.
Hope this helps!