4020 Sage Pay Error

11

After getting a response from SagePay I have found the following important notes:

    You can use Form/Server/Direct integration interchangeably on the same vendor's account, without needing to change settings or register anything
    The 4020 error genuinely is an IP restriction error and is not masquerading another error

The issue here was that the IP address of the web server (www.mysite.com), being on a VPS, turned out to not be the same address as the one used when curl requests were made. I made a test PHP page that mailed the IP in $_SERVER['REMOTE_ADDR'] to myself and put it on another server. I then used curl to grab that script and low and behold it was a different IP. Putting (a zero padded version of) this in to the Valid IPs section in My SagePay control panel (logged in with the admin account) it sprung to life immediately.
Update

You can use:

curl icanhazip.com

Like so:

[user@host ~]# curl icanhazip.com
177.12.41.200

to display the correct IP to use, from the command line of the server you're hosting the web site on, instead of uploading files and all that malarkey.