Does it have to be a solid wall, Tansy? If you have the space for a deeper wall how about getting more stone and building two dry-stone walls with the space between filled with soil and plants growing on the top. I would suggest that the soil should be at least six inches wide at the top and that the walls should slope in slightly, rather than being vertical, so that the stones rest against the soil.

Quotes might seem expensive, but if they're quoting for doing footings as well as rebuilding a wall, they're probably not. Hardworks like this in gardens always are more expensive than one expects, partially because of the materials required.