I've just tried with multiple draw calls using the same VBO and it appears this
does also cause the error. However, it appears there has to be 3 or more calls
to cause the error (1 and 2 calls produce the expected output).

Apologies for that. It was the way my application works that led me down that
path.

I currently do not have a simple program that demonstrates the error. I have
amended the application I've been writing to call the test shaders as I was
trying to work out what was wrong with my application. I could probably create
something. What is the usual format to provide this in?

Thanks.