I tried getting the logs but couldn't really see anything of value in them.
Tried many many things, with no luck, but finally have managed to resolve the issue:
1. Created a new control library project
2. Copied my existing source code to the new project
3. Removed all references to the old project from the solution
4. Added the new project to the solution
And surprisingly, the errors have gone! Same source code, same configuration. So I'm wondering if somewhere there was some entry cached in JustCode or Visual Studio? I have tried doing a diff on the .csproj files but apart from a couple of changed GUIDs they look more or less identical.
Anyway, the issue is now resolved (I have a couple of other things to check but will do them in a different thread!)
Thanks for your help, and sorry I have not been able to shed any real light on the original cause of the problems.