On my previous installations of Linux Mint, the wireless network has been automatically detected and all I had to do was enter the network key. It seems that with Linux Mint 13 XFCE I needed to give more information in 'network connections' to get the wirless network set up correctly. All I needed to do was add a wireless connection under the 'wireless' tab and enter the SSID name and the network key and that was it done. This sounds like a really obvious thing to do but as I hadn't done it before (except to enter the wireless network key) I didn't think to look.
I also forgot that when I was installing Mint 13 I remember seeing the option to connect to a wireless network as part of the setup process and several networks were listed so it was working all the time, it just needed the details.
Thanks for your help and I hope this will help others in same situation.