Thanks for your help Remco.
Numpty award goes to me.
I was so focused on checking the actual security permissions I did not notice that the file was blocked.
I've unblocked all the files in the directory just to make sure.
Now I've got the following error -
<description>CreateInstance failed for package [nCrunch.VSIntegration2010.CrunchPackage, nCrunch.VSIntegration2017, Version=3.11.0.9, Culture=neutral, PublicKeyToken=01d101bf6f3e0aea]Source: 'mscorlib' Description: Could not load file or assembly 'file:///C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2017\nCrunch.VSIntegration2017.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
System.IO.FileLoadException: Could not load file or assembly 'file:///C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2017\nCrunch.VSIntegration2017.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
File name: 'file:///C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2017\nCrunch.VSIntegration2017.dll' ---> System.NotSupportedException: An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See
http://go.microsoft.com/fwlink/?LinkId=155569 for more information.
 at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
 at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
 at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
 at System.Reflection.RuntimeAssembly.InternalLoadFrom(String assemblyFile, Evidence securityEvidence, Byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm, Boolean forIntrospection, Boolean suppressSecurityChecks, StackCrawlMark& stackMark)
 at System.Reflection.Assembly.LoadFrom(String assemblyFile, Evidence securityEvidence)
 at System.Activator.CreateInstanceFromInternal(String assemblyFile, String typeName, Boolean ignoreCase, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, Evidence securityInfo)
 at System.AppDomain.CreateInstanceFrom(String assemblyFile, String typeName)

System.NotSupportedException: An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See
http://go.microsoft.com/fwlink/?LinkId=155569 for more information.</description>
Should I do what it suggests? Or is there another suggested action?