As I took the front undertray off this morning so as to drain the radiator, one of these dangled down on each side. What the heck are they? I've been disassembling topside so may be related.

 

They are antennae for the TPMS to send signals to the control unit from the wheel sensors.

 

Huh! Well what do you know? Thanks for that. So they just sit on top of the plastic front undertray?

 

They should be secured to the back of the wheel arch lining so they're close to the wheel assembly as it rotates.

 

This image from the workshop manual gives a better idea of how to mount them - look for the curved mounting points that the antenna clips into at the rear of the bumper undertray.

 

Thanks Dave,I'm impressed that you could find anything useful in the factory online manual!

 

Yeah, the online manual isn't great. There's some steps I mention in my engine rebuild thread that it gets outright wrong, and where the person writing the manual clearly just looked at it and guessed, or where it tells you you need to remove something but in fact it's not always necessary. The other thing it took me far too long to learn is that often for a particular task it will tell you to refer to another section for removing some complex hard-to-access part, but that doesn't mean that you actually have to follow the entire removal process of that linked section - it's just to give you the information you might need to perform the task at hand.