Finally worked out what the problem was... nothing to do with ShaderForge. 

For some unexplained reason, the project on my Mac (copied from the PC version) had defaulted to using a Vertex Lit rendering path. I worked it out by going into the ShaderForge example scene and noticing that all of the shaders were displaying black materials, not just mine. So I knew the problem had to be an Editor setting. 

Changing to Forward, sorted it out.

Thanks Joachim. I guess I must have copied that project from my Mac and had forgotten. I ended up importing ShaderForge and recompiling the shader. That did the trick. Thanks for your help.

I tried that but I'm getting the following error: 

""

Shader error in 'circularFill': Parse error: syntax error, unexpected $undefined, expecting TOK_SHADER at line 1

then a warning:

Shader warning in 'circularFill': Shader is not supported on this GPU (none of subshaders/fallbacks are suitable)

Both projects a re using the same build platform, the second project is actually just a copy of the first project folder but with ShaderForge Deleted

Am I doing something stupid?

Can I add to this as a request? Legacy Rendering Path is incredibly useful for mobile VR and I find I use it a lot. It's a shame I can't use ShaderForge for this kind of work.