Some further updates for anyone still struggling, like me.
Since the solution file created by version 3.0.0 seemed to generate a VS2008 file, and I was trying to open using VS2005, I decided to downgrade my version of SharpDevelop to 2.2. Still got the same error, but the solution file created by SharpDevelop can now open & shows the project as 'unavailable'.
It would appear the conversion has only part completed, with certain parts not possible to convert. All files have been renamed to .cs, some code conversion has taken place, project file renamed to .csproj but the page directives on the aspx pages still indicate VB as the language and the code behind filenames as .vb
The slightly more interesting factor was when I renamed the .csproj file back to a .vbproj file - which suddenly allows it to be opened successfully in VS2005. Despite this not all is well as the code behind files now seem disconnected from the aspx pages they relate to, so choosing to open the code behind while an aspx is selected simply results in the aspx being opened.
I have now tried creating a new C# project and bringing the converted files in. This seems to be working more successfully although there are a great many errors where the conversion process hasn't added references/using statements (which may be intentional). It came as a surprise to me that with the page directives still indicating C# and .cs endings they still link to the correct code behind files in the newly created project.