I had a good chat with our specialist about the glow plug light coming on.
Just ask yourself one question. Does the car start properly in cold weather? If the answer is YES, then don't touch the glow plugs. If the answer is NO, then get it investigated.
He advised me that at some point in their life their resistance value seems to change a bit, which sets off the warning light, but that when they removed them from other cars they found nothing wrong, so their advice is to ignore the light as long at its not on all the time.
Ours does bring the light on now and again, but at 122,000 miles still has the original glow plugs, so if the car is starting properly, leave well alone. That advice given by two Alfa mechanics with well over 40 years Alfa experience between them, so I'm not going to argue.