Thanks petski :)

I removed mtop on my system (I'd previously set the root pw to '' as a temporary workaround)
    pete@lap043:~$ sudo apt-get remove mtop
    [sudo] password for pete: 
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    The following packages will be REMOVED
      mtop
    0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
    After this operation, 229kB disk space will be freed.
    Do you want to continue [Y/n]? Y
    (Reading database ... 214284 files and directories currently installed.)
    Removing mtop ...
    ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
    Processing triggers for man-db ...
    pete@lap043:~$ mysql -u root -p
    Enter password: 
    Welcome to the MySQL monitor.  Commands end with ; or \g.
    Your MySQL connection id is 63
    Server version: 5.0.67-0ubuntu6 (Ubuntu)

    Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

    mysql> SET PASSWORD FOR root@localhost=PASSWORD('');QUIT;
    Query OK, 0 rows affected (0.03 sec)

    Bye
    pete@lap043:~$ sudo apt-get remove mtop
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    Package mtop is not installed, so not removed
    0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Set the root password again, and then ran your instructions:
    pete@lap043:~$ sudo sh -c 'echo deb http://ppa.launchpad.net/petski/ubuntu intrepid main > /etc/apt/sources.list.d/LP77980.list'
    pete@lap043:~$ sudo sh -c 'echo deb-src http://ppa.launchpad.net/petski/ubuntu intrepid main >> /etc/apt/sources.list.d/LP77980.list'
    pete@lap043:~$ sudo apt-get update
    pete@lap043:~$ sudo apt-get upgrade

And then installed mtop without needing doing any changes in mysql! :)
    pete@lap043:~$ sudo apt-get install mtop
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    The following extra packages will be installed:
      libconfig-ini-simple-perl
    The following NEW packages will be installed
      libconfig-ini-simple-perl mtop
    0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
    Need to get 60.6kB of archives.
    After this operation, 295kB of additional disk space will be used.
    Do you want to continue [Y/n]? y
    WARNING: The following packages cannot be authenticated!
      mtop
    Install these packages without verification [y/N]? y
    Get: 1 http://ppa.launchpad.net intrepid/main mtop 0.6.6-1.2ubuntu2 [52.1kB]
    Get: 2 http://gb.archive.ubuntu.com intrepid/universe libconfig-ini-simple-perl 0.02-1 [8516B]
    Fetched 60.6kB in 22s (2754B/s)                                                                                                                                                                                                             
    Preconfiguring packages ...
    Selecting previously deselected package libconfig-ini-simple-perl.
    (Reading database ... 214301 files and directories currently installed.)
    Unpacking libconfig-ini-simple-perl (from .../libconfig-ini-simple-perl_0.02-1_all.deb) ...
    Selecting previously deselected package mtop.
    Unpacking mtop (from .../mtop_0.6.6-1.2ubuntu2_all.deb) ...
    Processing triggers for man-db ...
    Setting up libconfig-ini-simple-perl (0.02-1) ...
    Setting up mtop (0.6.6-1.2ubuntu2) ...

No more error messages, or changes required to mysql :) It works great!