OK been around a lil, not sure if this one is on the map or not (there is a wall near that's marked, but it doesn't look like the one I'm thinking of). There's a wall at the back of St Leonard's Place car park, with a square window-like hole in it. Lots can be done, though I'm just scratching the surface of it
Plus, bordering the park near the York racecourse there's a wall from the road (above) down to the park. The park gently slopes, where the wall stays at the same height; and below is mostly just grass (with some sets of steps to try more precision jumps on also). So, you can practise techniques at any height you feel comfortable at, and push yourself only a little higher each time if you wish.