Ubuntu Error 2002 “Can’t connect to local MySql server through socket…”

I am trying to access MySql on my Ubuntu 10.04 LTS (lucid) system. I am aware that there are a lot of results out there for this error but none seem to be helping me. I am a novice on Linux so any help might need to be spelt out.

When running

mysql -u root -p

at the command prompt, I receive the error

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (13)

The sock file definitely exists; running

sudo ls -la /var/lib/mysql

includes the line

-rwxrwxr-- 1 mysql mysql 0 2011-11-03 19.05 mysql.sock

and running ls on the parent directory looks like this

sudo ls -la /var/lib
drwxr-xr-- 5 mysql mysql 4096 2011-11-03 18.14 mysql

I think MySql is running;

sudo service mysql start

results in

mysql start/running

And the my.cnf file (located at /etc/mysql/) includes the line

socket = /var/lib/mysql/mysql.sock

under the [mysqld] section.

Any pointers would be greatly appreciated. This site is live and since it is hosting a WordPress installation, not being able to access MySql completely breaks the site!

EDIT

Output from comment;

ps -ef | grep mysql
mysql     1869     1  0 20:15 ?        00:00:00 /usr/sbin/mysqld
pete      1953  1713  0 20:36 pts/0    00:00:00 grep --color=auto mysql