As noted on bug 64596, this is probably what is blocking me submitting patches.

From related discussion on https://bugzilla.wikimedia.org/show_bug.cgi?id=65548#c3

I think the best fix for this would be to update all users that have
550(svn) as their primary group to have 500(wikidev) as their primary group.

Following that one of two things should happen, either all files owned by
group 550(svn) should be changed to 500(wikidev) across all of labs, or
probably more rationally all users in the 500(wikidev) group should be added
to the 550(svn) as a secondary group. If the later action is taken the
script that creates new users in ldap should also be updated to add all
future users to the 550(svn) group as a secondary group.

OK, done:

1. Everyone with a primary group of 550 now has a primary group of 500 instead
2. Everyone who used to have a primary group of 550 is now an explicit member of the 'svn' group, 550.

#2 should ensure that everyone can still read their files. I'm braced for labs users to inform me otherwise...