Well I've managed to make a little progress, but I have no idea why it started moving along.
Firstly I decided to download an older version of DNN, 4.8.4 (as I have this version working happily elsewhere) I also thought it might be a good idea to launch FileMon and see if I could find out more information about the web.config access error. This resulted in the site redirecting to the "under construction" page, I then navigated back to the InstallWizard.aspx page and was allowed to enter.
Now that I'm in I'm stuck again on the Database page, I get 3 errors,
Index #: 0
Source: .Net SqlClient Data Provider
Class: 11
Number: 4064
Message: Cannot open user default database. Login failed.
Index #: 1
Source: .Net SqlClient Data Provider
Class: 14
Number: 18456
Message: Login failed for user 'ALTAVENTEDEV2\ASPNET'.
Index #: 2
Source: .Net SqlClient Data Provider
Class: 0
Number: 5111
Message: File activation failure. The physical file name "C:\DNN\App_Data\Database_log.ldf" may be incorrect.
I launched the server browser and added ASPNET to the logins and also added the ASPNET user to the 3 SQL user groups, whether these are necessary or not I have no idea as I've never had to do this before. Unfortunately still no luck, so I attached the database file manually and added the ASPNET login as dbowner. Unfortunately this hasn't helped either.
Any ideas on how I can gain access to the db for ASPNET?