Can't connect to database server

I am trying to set up 2 new sites, and both keep coming up with the same error upon install.

I altered the name of the wp-config.php file to wp-config.bak using filezilla so I could manually insert the database details as I have had issues at this point in the installation previously.

I’m on 1&1 and they say the databases are ready to use, but when I click ‘submit’ I get this error message;

Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at db2751.oneandone.co.uk. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure that you have typed the correct hostname?
Are you sure that the database server is running?

I am sure the username and password are correct, and 1&1 say the server is ready to go.

What is wrong?

Thanks in advance for your help.