Logging in to iMac desktop - forgotten password

I shut down my iMac (moved house etc) and have not used it in over 9 months... when I booted it up the other week, I was unable to log in, and cannot remember the password (and the hint was too cryptic for me to remember the really clever thing I thought of at the time)

I have reset my Apple ID, but the machine obviously has a local password and its not letting me past the login screen...

Am I missing something? How do I bypass or reset the login at startup?

Thanks