What's this thing on the rear lower control arm?
 

Probably a stupid question but can anyone tell me what this thing is for that is attached to the lower control arm on the rear of the car (and only seems to be on the passenger side). I managed to shear off the bolt that holds it on to the control arm whilst replacing the stock arms with these Sumit ones today, and I'm just curious exactly what this part does and why it would only be on one side.

One of my friends says it might be for automatic headlight adjustment when the car moves up and down, but I can't find any mention of it in the service manual for the car.

Attachment 102575

here's what it looked like detached from the swing arm:

Attachment 102574

There is a headlight leveler as Cusco sell an adjustable piece for it. Best to get a UK dealer manual to learn more about it.

Thanks - went to a UK dealer today and they couldn't find it mentioned on any of the diagrams of the lower swing arm, but we did eventually find it listed in the electronics section and it is just called "auto levelling system" or something like that - doesn't actually specifically mention lights but they agreed that was what it was for. Unfortunately you can't just buy the part that I broke so I would have to buy the entire thing and that is about £130... so we're going to try just drill out the broken bolt and weld on a new one.

It is indeed for automatic headlight levelling- it's a requirement of hid lights in the uk market along with headlight washers. Other markets vary so may not have them.
Not sure how the lights 'fail' when the sensor isn't working or connected, probably low so if you find your lights pointing just in front of the bumper then that will be why!