While underneath the car I noticed what I think are the height sensors are disconnected, I assume this is because the suspension has been lowered, should I leave them as they are or re-connect them? Does lowering the suspension make these sensors not work correctly as the height is now not what it should be?

 

You will likely 'screw-up' the HID headlamp aiming.
If they are set correctly now, I would leave them alone.

 

You are probably looking at the height sensor, input to the HID headlight leveling system. I would reconnect it, I believe these are normally required for proper "legal" operation.

 

IF the suspension height is altered (lowered) the aiming MIGHT be raised too high? blinding oncoming traffic and not light the road properly?

If the car is lower PROPORTIONALLY (both front and rear equally) then all would be correct and reconnecting OK.

 

If it is like mine you will have great difficulty finding the control arm as it is no longer available. I had to build one from a piece of metal.

 

If the only thing effected by this sensor is the beam height I think I will leave well alone, the control arm is still there but the bottom end is not connected. The car sailed through the annual mot test without the lights being mentioned.

I find it odd that these cars would need to be fitted with these sensors given how unlikely it is to have more than two people in the car, or anything of great weight in the boot/trunk, how far out of level is it ever going to be?

 

To my knowledge, leveling is part of the system. HIDs are so much brighter than the old halogens. I believe regulators approved HIDs on the condition manufacturers make a best effort at avoiding "blinding" oncoming traffic if the vehicle's balance was off. I suppose you could still load up the trunk with serious weight and get the headlights to point too far up. There should be another sensor somewhere on the rear.

FWIW, I do not believe the design of our suspension takes kindly to being lowered as negative camber quickly develops to excess. Assuming your lowered springs lower front and back evenly, I am not sure how that would prevent the sensors from doing their job as designed. Remember, even if you are disconnecting the arm, the sensor is still sending a reading to the controller. So unless you are somehow pinning the arm to a known good location, your leveling is likely off. Unless of course someone disconnected the levelling altogether. FWIW, I still believe you would be better off with the system connected and operational. You already paid for it, in a way, so you might as well use it. Your car, your call.