OK well the progress on this is that I found out that I had to save the form as a DLL, so I did that but the source project was still part of my overall solution. This came up with all sorts of wonderful errors that are totally incomprehensible to me.

I moved the ParentClass project out to a separate solution, rebuilt it and magically now I can add one of these forms to my proper project.

Except...I cannot get the RegisterPrimaryBizObj method to be recognised - can you tell me which namespace it is located in please?