Just to close this off, here is the offending brake pad and as @rhino666 predicted it has a groove in it.
Also, as @RichardFX predicted both the inside pads were only making contact for about 1/2 to 2/3 of their width. Sadly @BrilliantBlackBirdy was also right and both rear calipers were faulty and need replacing, so I will get the discs done at the same time. That saga can be followed here Help needed trying to replace rear brake pads.
