[diospyros]

Late entry from me... though this won't solve your problem as will just create more spoil... but I always think there is something lovely about a raised pond where you can sit with your nose almost in the water watching the pond skaters!  Because the back of the pond would be level with the ground, wildlife could still get in.  You could use a rigid liner so the weight wouldn't be on the wall.

Just what i would be tempted with!

[madcat]

Southwest facing, poor soil and well drained - I'd plant it up with woody herbs.  Lavenders, rosemary, sages (purple, green, tricolour), curry plant and southernwood even.  Would smell gorgeous.  You could still do that round your bench - assuming that you are happy to sit among the buzzing of bees and fluttering of happy butterflies.

[Mrs Bee]

some prostrate rosemary would take the edges off the wall as well.