It’s not really hard to do, but I admit it is a lot of work (luckily this isn’t a big artist). You’ll have to go to the releases and change the release artist from the wrong Julian Mendelsohn to the right one. Here’s the MO:
1. Create a new artist Julian Mendelsohn with a descriptive disambiguation comment.
2. Copy the URL of the new artist.
3. Edit a release
4. Edit the release artist and paste the URL into the field. The release editor will load the correct artist.
5. Check the track list tab to see if it uses the correct artist now (that should be automatic if the track artist is the same as the release artist, else you’ll have to change them manually).
6. In the recordings tab, tick the check box to update the recording artists.
7. Leave an edit note and enter your edit.
The above looks like a lot of work, but once you get a hang of it, it will be fast.
I’m afraid you have to change relationships one by one though, but again you can paste the URL to the correct artist to make selecting it faster.