Incorrect permissions on dkim selector file

My dkim selector file is currently owned by sympl:sympl, with permissions set to 660.

I received the following error in my logs overnight:

2019-06-27 06:39:42 1hgN8H-0005FM-Rw failed to expand dkim_selector: failed to open /srv/gentlysympl.gentlyhosting.uk/config/dkim: Permission denied (euid=105 egid=109)

What should the permissions / ownership be set to? The uid / gid referred to in the error are both Debian-exim. Can sympl automatically adjust these permissions if a specific set are required?