Problem solved! This isn't going to help anyone apart from myself and will enable closure of this thread.
Dealership contacted the previous owner and was informed that the left fender was replaced due to something dropping onto it in a garage accident - cosmetic only. I pulled the wheel housing to have a good look round and everything looks sound. No signs of frame repair or wires disturbed etc.
Since the fender was replaced the skirt was removed to access the two bolts under the fender.
Looks like the wrong clips were used to re-install the skirt back on.
Ordered the correct ones, replaced, installed and now good as new.
Side skirt is solid and no longer sticks out.