Never mind I aswered my own question, with some hints from other messages on the MSDN board.

I modified the ACC.csproj file for each of the partial class file from:

<Compile Include="frmAcc.Dev.cs">
<SubType>Form</SubType>
</Compile>


TO:

<Compile Include="frmAcc.Dev.cs">
<DependentUpon>frmACC.cs</DependentUpon>
</Compile>


This also placed the tabbed files in the solution explorer under the main file frmACC.cs

Hopefully this stays put as the .csproj file is generated by the IDE.

Thanks to anyone who took the time to read this post.