It's a bit dissapointing that I didn't get a reply to my problem.
Howevere after numurous attempts I manged to get it solved and find out the problem.
Instead of using the silent install of SQL Express through Web Platform Installer, I done the following to resolve the problem:
Attempt to install SQL Express through WPI - where it prompts you where it will download the .exe/.msi which is a link you can click on. So I downloaded this to my desktop.
Then I went through the installer myself going through the steps to make sure everything was OK.
The error I was having that for the Network Service account for the SQL instance was having a permissions error.
So I uninstalled SQL Express (even though the install fails it still adds the instance somehow?!)
I went through the installer again and for selecteing the Network Service account. I manually selected the Network Service account from my local machine.
What it seemed to be doing was selecting the network service user from our works DOMAIN that my machine is joined to over VPN.
Selecting the local network service user managed to fix the problem.
Hope this helps anyone else out that ever encounters this problem.
Warren :)