Thanks Marc,
It was indeed the Hint Path.
<Reference Include="RFXControls">
<HintPath>..\FrameWork\Component\RFXControls\bin\Release\RFXControls.dll</HintPath>
<Private>True</Private>
</Reference>
<Reference Include="RFXControls" />
<Reference Include="System" />
<Reference Include="System.Data" />
<Reference Include="System.Data.Linq">
<HintPath>C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.Data.Linq.dll</HintPath>
</Reference>
The above is what was originally in the problem project file. The lines below are from a clean project, correctly picking up the DLL (RFXControls is one of our own). I made the path absolute, but that made no difference, then I spotted the second entry for RFXControls. Once removed, it picked up the DLL.
<Reference Include="RFXControls">
<HintPath>D:\Oxygene\FrameWork\Component\RFXControls\bin\Release\RFXControls.dll</HintPath>
<Private>True</Private>
</Reference>
Odd that you can’t (as far as I know) see these paths visually in VS anywhere, but one more issue down. Thanks again.