I have just been comparing my wife's working wireless settings with my own I notice that my TCP/IP setting is set to 'manually' whilst hers is 'using DHCP' when I change this setting on my laptop the IP address changes from the correct one that I put in (given to me by my server provider) to a 'self-assigned IP address'. Could this be the problem and do you know how I can resolve this?
If it's set manually, then it's a static IP. Wireless routers allocate IP addresses dynamically using DHCP - and those IPs would be in the range of 192.168.xxx.xxx. Any IP provided by the ISP is likely a public IP and isn't intended to be used with computers connected via a router.
Setting your Airport adapter for DHCP is pretty straightforward. Go into System Preferences => Network => Choose Airport in the left window pane. Click Advanced button => Click the TCP/IP tab, and change the "Configure TCP/IP v4" field to "Using DHCP". Click OK.
If this doesn't solve the issue, it is likely related to your wireless security settings. You may want to first try removing the wireless network from the list (System Preferences => Network => Choose Airport in the left window pane. Click Advanced button => AirPort tab => find the network name in the list, click to highlight and then click "-"). Then, try to rejoin the network, it should prompt for a password. Assuming the password is entered correctly, you should connect and be ready to go. If it does not, you may need to access your router's configuration webpage by connecting via Ethernet to the router and checking wireless security settings. It should be set to use WPA or WPA2 (do NOT use WEP).
Specifics on how to access your wireless router's configuration and navigating through the menus are contained in its manual.