It seems that I was right, as soon as I removed the password, HAProxy could see the databases. I've gone through the notes for HAProxy, and there doesn't appear to be any way to specify a password.
Whilst I appreciate it is only using that user to connect / disconnect this still seems like a bit of a security issue, as I now have a user in my database with no password. The best I can do for now I think is lock down the IP Address for the user as tightly as possible.
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