Well, I've gone the following route:
It seems that the best mounting point by my perceptions is to the underneath of the left hand rib of the seat. Because this is double layer and sealed, and I didn't want to drill lots of holes in the seat to access bolts, I had to improvise with a home made bracket.
Took some 2mm ply, cut it to a rectangle the length of the bottle cage mounts, and glued two bolts in place on this. Then built up a hollow box around the nuts, ending up with a 9x1.5cm section that the bottle cage mounts to. This was then glued with plastic steel to the bottom of the seat rib.