Sorted! Don't know why I didn't think of it earlier - couldn't see the wood for the trees.
I googled for the clearest pics of chassis' and downloaded, zoomed and printed off two of them. With schoolboy maths and scaling, I worked out the distance from the rear seat mounting holes to the silencer boss on both and struck the average, and with a hole saw matching a blanking grommet I had laying about, I drilled the inner panel looking for the boss and was only 5mm out from the centre (phew!). Then drilled through the boss and through the fibreglass, then holesaw'd from the outside.
My biggest fear was that because it left the factory with an under-slung exhaust with all the extra tabs for that welded to the chassis, they never welded the boss on for a side exhaust.
She'll be back on the road this afternoon.