MAMP Pro failing to launch (mysql)
Due to trying to get my local server up and running again, I installed and got working XAMPP, which then had a knock on effect of allowing MAMP Pro to display a decent error message (instead of continually working) and allow the options on the interface to be active again. MySQL was running but Apache couldn't launch. Also, although I could change the settings on the MAMP interface, none would save. looking at permissions and ownership, the app and files were owned by root (but not sure that had any refection on the issue).

@bassplayer7 gave me pointers to the uninstaller (something I didn't know existed), which due to permissions couldn't actually do anything. So I found these excellent instructions:
https://stackoverflow.com/questions/11855247/mamp-pro-restore-root-user-uninstall
Which cleaned it off completely.

I then did a system restart, re-installed MAMP Pro completely fresh, and it all worked first time. Albeit without any databases, hosts or settings.

You do have backups, right?