Right, progress. Of sorts.
Quick insight to the CANBUS magickery on the Barge.
The engine CANBUS circuits have two connection nodes... one under the passenger foot well, and one behind the dash.
The ESP module connects to the foot well one, and the engine (and instrument cluster) connects to the one behind the dash. These two nodes are connected via a plug also under the passenger foot well.
There is also a further CANBUS power distribution block under the steering column, although the only things plugged into it are H and L CANBUS wires, ie no traditional power and no official connection to the ESP module... it would appear to connect the primary modules... engine/instruments/shifter/EIS/SAMS.
So, to the progress...
ESP H and L CANBUS shows no breaks all the way to the cluster plug. I didn't pull the dash to access that node, but did check for continuity at the footwell node, both sides of the footwell plug and the cluster.
The single diagnostic wire also shows continuity all the way to the cluster and OBD2 port.
Now here comes the hiccup.
I also found continuity between the ESP H Line, both the dedicated CANBUS wires (between the instrument cluster and the CANBUS power distribution block) and a feed to the cluster from F22 in the front left SAM.
To confirm, there's definite continuity between the ESP module H line and F22 when tested at the SAM, (they all live in the same SAM so easy enough). F22 provides permanent power to the Nav system. I had previously found damage to a couple of the wires at the power distribution block which were patched up along with undoing the starter button bodge the previous owner had had done
I suspect, that there's either a pinch or damage on the cluster loom, which may well require the dash to be pulled to investigate/rectify.