PROBLEM SOLVED! When I fitted my bluetooth audio receiver, I removed my displays and scratched the heater panel. I bought a 2nd hand one from eBay for £25 to restore it to former glory and fitted it today. The two screens are now exactly the same brightness, so it looks like it is a fault. My original heater screen was brighter than the radio one, but now it is dimmer and the two are matched perfectly and stay matched when I change the brightness level on the headlight switch. The part I bought has a different part number to my original one and was manufactured 2 years earlier but so long as the front panel is the same, the rest matches up fine despite the part numbers. Hope this helps you out.
Alan, Nearly bought the XKR-S, but in the end went for the Supersport, it is 5 litre Supercharged and is a lot quicker than both the Aston Martin DB9's I owned, so i'm pleased with the power. ATB, Trevor