Spot the difference: the solution
If you have not seen the original post, you might want to have a look first, before reading on.
The difference is in the lower right corner, in the leaves of the plant at the bottom of the wall. In the original picture they looked as if they were a continuation of the wall, which in turn made the wall look bent. Now there is a clear difference and the wall looks straight-ish. (The real one isn’t too straight either.) For the same reason I also softened the line where the wall meets the walkway.
And the location of the original wall and garden? Try to find it next time you visit St Albans in summer.