There used to be a tree in the corner. It had been cut down before but the stump had sprung back into life again and I'm pretty sure that this is what has caused the damage. I've now cut the trunk and the new growth down again and the stump is still in the soil. My plan is to remove the stump, dig a trench behind this top section of wall to take off the pressure and then dismantle and rebuild this two foot upper section.
Having seen some other local gardens where the soil has pushed the retaining walls into gravity-defying distortions I suddenly wondered whether the soil and roots might actually be holding it all together? In which case I suppose the wall ought to be dismantled first - to avoid stones dropping down into the next level.
Any thoughts about which way to approach this? Is it within the scope of an amateur bricklayer like me or ought I call in someone with more experience?