What northy said is right wobba, the backing plate on the left hand pad looks warped and this is the one that will fall out and render one of the brake circuits useless.
some of the retainer sliders have holes in each end but the hole at the left hand side needs the spring clip to stop the retainer moving to the right and falling out. when it moves the other way it hits the bolt head and stops but that does not matter as the outer pad should be bolted on.
I suggest you spend 7 quid or whatever on a fitting kit as you get new retainer sliders, anti rattle springs and more importantly new spring clips that are still springy and wont fall out (and save you cleaning the old ones) and it will all move freely for a while again.
I think the caliper setup on these has allways been a pain in the backside and mine allways seem to be binding on after the winter salt. I speak from experience of a pad falling out when my ex was driving, luckily in town not barrelling down the A1!