I tried a midfield takeoff yesterday from YBCS (Cairns). I got to the midfield - and ATC contacted me and gave me instructions for a midfield takeoff. Then told me to line up and wait. Which I did - but I never got clearance to takeoff. I contacted Tower and they just repeated (in what I thought was an impatient voice) just to wait.

To get to the actual end of the runway I would have had to back taxi down the main strip and it was a fair way. And I had already been given instructions to line up and wait.

In the end I gave up and just took off, but ATC just ignored me after that.

I am guessing the aircraft did not get near a magic 'mid field' marker so did not trip the next phase of flight. What should I have done in this situation?

I would need to see the log file for that flight to be able to see what happened.  You can email it to me (admin @ pilot2atac.com).

Log files are located in:

C:\Users\<UserName>\AppData\Roaming\P2A_200\Logs

where <UserName> is your PC user name

A new one is created each time you start Pilot2ATC, so if you take the most recent one when you have the error, you should have the correct one.