Later this afternoon I moved inside the garden to remove the ivy from that side of the wall, and also to make a start on cutting back the boundary hedge. I took this photo showing that corner of the garden when it was almost totally dark, so apologies for the not great quality:
There's still a bit of ivy to clear off the inside of the wall, but I'm not going to remove any more from the far end of it until I have got advice as to how to best stabilise the stones.
I've made a very small start on tidying the hedge itself. It looks as though down through the years several attempts have been made to block the hedge with chicken wire, plastic screen-like wire, and even (now completely rusted) chain link fence. None of these seem to have been erected on fence posts; more just pushed against the hedge and tied back with bits of wire. I hope to remove them all, as they otherwise will prevent me from cutting the hedge as low as I'd like to. But I can see them being difficult to remove, as brambles, ivy, and the hedge itself, have grown through them in lots of places - and the bottom few inches of them are buried under the accumulation of years and years of rotting leaves...