Restore the Thieves guild - Special Mission in Solutide

I seem to be having some trouble getting to the special mission in Solutide.

I've been keeping count, multiple times, of the number of missions I've been doing for Vex and Delvin.

At the moment I've got a count of 13 solitude missions. 5 of those for Vex, 5 for Delvin, and the other 8 were a mix (I wasn't tracking whether it was a Vex or Delvin mission at that point).

I'm yet to receive the special mission for Solutide and it seems to be going on forever.

Is this a bug or have I missed something fundamental?

This is a glitch, and you aren't the first one to experience it.

If you are on the PC, you can force the quest to start by using this console command:

setstage TGTQ02 10