Well, I simply could not find this (for me) mythical "Use dynamic ports" option. I have post screenshots.
On a more constructive note, I believe that the port numbers are to be found in the solution file AND CRUCIALLY cross referenced against the IIS Express config file
C:\Users\<username>\Documents\IISExpress\config\applicationhost.config
I tried editing the port number in just the solution file but strange things happened. I propose (no time yet) that it needs a consistent edit across both the solution file and the config file.