I tried your suggestions - ASP.Net was correctly installed and there were no obvious issues with permissions.
I did a couple of re-installs and tried various options.
The two changes I ended up making that resulted in me being able to see the Deployment Manager site were :
1. Granting IUSR and NETWORK SERVICE accounts access to the location for the physical files in the "Portal" directory
2. Changing the Authentication settings on the Deployment Manager website in ISS to enable Windows Authentication (it seems this was disabled by default)
I will let you know if I come across other issues but this was obviously the first and most important one ie. being able to actually get to the Deployment Manager site.