Thanks to everyone for your replies! Looking more through the Bentley manual, I saw that there are in fact three panels for the assembly. It sometimes helps to do more thorough research...
I'm not sure if I'll replace them at this point. I don't know how long they've been off the car, since they weren't there when I bought it a while ago and I haven't noticed any problems so far.
This reminds me of the issue with the engine under-tray on my Porsche 993; more of a nuisance that provides no great benefit.