I decided to strip the caliper and see what if anything was happening. There was slight grooving on the inside pad where the rust had stopped the pad from contacting the full width of the disc no problem with the Pistons. I took of the cap to the master cylinder and covered the top with a cloth and refitted the cap loose. Used a clamp to push back the pistons and then checked for any problems but there were fine. Cleaned the pads and refitted them then tested the brakes and all ok. I decided to look at the service history prior to my buying the car in September 2012. It had done 11,200miles and it seems that in 2011 it had done next to no miles at all only 460 miles. My conclusion is that the car had probably been stood with no use for a long period of time probably 8 to 10 months. I have posted a photo and it looks rather rusty under that side. I wonder if this could have caused the corrosion.
