Our reasoning and expectations behind this.
We originally bought a telescopic ladder to access the roof but have now fitted a straight ladder in runners under the roof rack.
Sorry, there are no construction photos.
Runners attached to the underside of the roof rack using u bolts,
the second piece of ally channel around first gives extra support and acts as a spacer.
Bolts through the top of the ladder act as the runners.
Each bolt has a piece of ally tubing just smaller than the channel and a large washer on it forming a guide. 
A hole drilled through the channel with a bolt dropped in stops the ladder sliding out of the end when traveling, we will probably replace this with a carabinier or long-staple padlock.
Rubber bungs for the ladder ends will also be fitted sometime soon.
The ends of the channel have been shaped to assist essay insertion of the ladder.
We intend to fit a bar underneath to prevent the ladder from hitting the roof when being loaded/unloaded (drawing showing the concept not to scale). We will post photos once fitted.
Currently, we are using the tow bar to stand to reach the ladder but eventually will fit a fold-out step of some sort to the bumper. We are also hoping to use a similar system on each side of the ladder underneath the top boxes for our sand ladders, More photos when we do.
More info & photos soon
In-use reviews.
The ladder weighs 5.5 kg + fixings at about 1 kg, so slightly lighter than the telescopic ladder it replaces and has permanent out-of-the-way storage. Things may change in the future when we fit the pop-top roof, but we are hoping to use the same system on the pop-top.
Pros.
Lighter than the telescopic ladder it replaces
Out of the way on the roof.
Cons.
Accessing the ladder is a bit awkward at present, but we intend to fix a fold-out step on the rear bumper.
Fixed length.
Extra weight on the roof.
Our conclusions.
Other related or affected parts.
4 Bar Roof Rack.
Sand-ladder rack.
Fold-out step on the rear bumper.
Pop-top roof,