Found my Multibit.key wallet back up, but I do not remember the password, what next? I created a Multibit wallet back up many years ago and it was saved as multibit.key. 
I am now trying to import this back into MultiBit Classic and it is asking for a password. I have forgotten the password. 

Is there anything I can try? Is brute forcing an option? What type of software would I have to use?

Cheers
You can try using btcrecover to attempt to brute force your password. However, if you have no idea what the password could be, then it isn't going to be able to help much.