Access only available as root

Have just set up a new Kubuntu machine (18.04).
Some how I've taken a wrong turn in setting up mysql and can only access mysql as 'root'.
In other words, in Terminal:-
:~$ mysql -u root -p gives "ERROR 1698 (28000): Access denied for user 'root'@'localhost'".
but
:~$ sudo mysql -u root -p
works fine
Any advice welcome