Behavior issue with one of my rescue dogs
 

In 2005 I brought my five dogs (4 rescue) to Canada from the UK. I have two dogs that were rescued after being strays and both were adults when I adopted them. It is one of these adults I am having problems with - he is about 11 years old and in perfect health with no previous health problems. He is 4th in the dominance line but has now got to the stage where he will not leave anyone alone, if it is not me he will follow whoever is in the house around like a shadow - he prefers to be with a human than playing outside with his mates. The following around has now got to be an obsession with him and even after being told repeatedly to sit down, he doesn't seem to be able to keep to it for more than a few minutes. Any advice would be appreciated?

If you have noticed a difference in behaviour then I would seriously consider a vet consultation. Animals cannot verbally of course let you know if anything is wrong..so we must rely on our professionals to find out.
At 11 years old...your dog can be signalling you that something is off. The dog is feeling insecure...or fearful. Could it be do to pain? Could it be due to a shift in sight/sound?
Please see your vet to rule out any medical issues. That would be my first step.

I agree with BenMax and would have him checked out by the vet. Major changes in behaviour without an identifiable trigger usually indicate health problems. Good luck!