If your IP is truly changing, then the ideal thing would be to get that sorted -- it's not a XenForo-specific problem in anyway as there are various apps that use the IP to help identify users. (We use it as an extra layer of security.) It's generally a well documented issue and why proxies should use "sticky sessions" if possible.
Otherwise, you can potentially add code to config.php that uses the forwarded IP. However, as this is spoofable, it's generally not recommended as it effectively undoes the security value of checking an IP.
Jake - I can log into the forum as a user/admin no problems - it's just when I try to access the AdminCP - Put in my username and password and i go straight back to a blank AdminCP login screen as if i had never tried to log in
Mike - I work for the Uk side of an international company with 15k+ employees in the UK alone - I've got no hope in changing Corporate IT policy - specially for my own "hobby" site