Thanks Spiderman/Donkin for the feedback.
Had a look underneath and other than the rear bumper section, rear light strip, etc, could not see any obvious major problems.
Still gutted, but considering the speed of the shunt I am surprised how the Spider back end energy absorbing took the brunt of the impact and limited the apparent damage. Its given me more confidence in the rear body strength, but still concerned about possible hidden distortion as you mentioned.
Contacted insurance company Saturday, but it appears to be a busy weekend for accidents (over 200 claims reported to them) they now say it will be Wednesday before they can get to my claim. I note your points made about "Insurance company approved repairs".
Now comes the waits and insurance delay hassles??