OK so I eventually worked out that I needed to add conditions to the references in the csproj file like this:
<ItemGroup Condition="'$(TargetFramework)|$(Configuration)|$(Platform)'=='net8.0-windows|Debug|x64'">
<Reference Include="RevitAPI">
<HintPath>c:\Program Files\Autodesk\Revit 2025\RevitAPI.dll</HintPath>
<Private>True</Private>
</Reference>
<Reference Include="RevitAPIUI">
<HintPath>c:\Program Files\Autodesk\Revit 2025\RevitAPIUI.dll</HintPath>
<Private>True</Private>
</Reference>
</ItemGroup>
<ItemGroup Condition="'$(TargetFramework)|$(Configuration)|$(Platform)'=='net481|Debug|x64'">
<Reference Include="RevitAPI">
<HintPath>c:\Program Files\Autodesk\Revit 2024\RevitAPI.dll</HintPath>
<Private>True</Private>
</Reference>
<Reference Include="RevitAPIUI">
<HintPath>c:\Program Files\Autodesk\Revit 2024\RevitAPIUI.dll</HintPath>
<Private>True</Private>
</Reference>
</ItemGroup>