Finally managed to take the Company Server running DNN to version 4.3.5. We had been experiencing problems with the file manager and permissioning users in new role groups. I had run into trouble trying to upgrade to 4.3.4, so had left upgrading the server until this new version of Dotnetnuke had been out for a while.
Once the site had updated and you were prompted to click on the link to return from the install screen to the main portal,I encountered the default page going into a redirect loop.
Turns out I had to not include the default.aspx and default.asp.vb files. I also avoided updating the web.config. but I think the default.aspx files were the cause of the redirection loop.
My boss is happy so all is good.