Retaining Garden Wall Crumbling Away

as it is holding back a raised foot path presumably owned by the council who own the wall as Iam sure it would have been put in by the same people who done the footpath to support the path, really the only way to repair is to start again and rebuild, bricks are porous if there are no weep holes to allow water out the brickwork over time will just disintegrate & crumble , the only thing holding it together was the render, the top will always remain relativly intact as water soaks down not up, fell you may have opened a can of worms that will come back and bite you. you will have to contact the owner of the foot path to see where you can both go from here.

Depending on the erosion I would take it all back to where it is solid and fill with a good mortar/cement mix and then re render. Your description isn’t the best of what’s up but I presume some parts are in bad condition and the majority good? If that’s the case then patching won’t be a problem. If it’s the opposite then maybe a re build would be considered. Hope this helps.

I think it would be a good idea to seek professional advice as trying to patch up more cost more in the long term. Dont think it will be a cheap fix