Think I have it sorted now. It was bugging me too much so I pulled the until apart again this morning. I had a feeling it was the self adjuster at fault, ie not adjusting both pads evenly so they made contact at the same time. This time I wound it in a lot further so it could really "adjust". Seems to have done the trick. Both the stationary clicking and braking clicking have gone. Lets see if it stays away!