Hello @cormac.seamus,
Welcome to the Community!
I apologize for the inconvenience this is causing you. I will suggest that when you are sure you are using the correct master password and is not getting accepted, try to access from a different browser or device if it does log you in, proceed to clear cache and cookies in the browser and restart the browser (close and reopen the browser) then login again and that should fix the issue.
In the case, that the master password is not accepted anywhere it means that the master password is wrong and you will need to recover the account.
Kind Regards,
Alejandra
LastPass Support
