Running GPEDIT.msc: Computer Configuration > Windows Settings > Security Settings > Account Policies > Password Policy
I am unable to change any of these policies as they appear to be locked (the icons have a little padlock against them and when I open properties all the options are greyed out)
For what it's worth I'm trying to reduce the password history value.
I'm logged in as admin on the domain controller for the network.
I've googled but can't find anything that fits, anyone got any ideas?
The problem is that you're trying to manage a domain controller using the Group Policy editor to edit the local group policy settings, which isn't going to work. You need to use the GPMC to edit the default domain policy that is linked to your domain. The lock icon is a clue that the policy settings you are looking at are being set via domain policy, not local policy.
Have you tried the Group Policy Management Client?
From: http://technet.microsoft.com/en-us/library/dd367853%28WS.10%29.aspx