Yep, and it built that. The project file contained libs for .a (linux), .lib, and I think one other (using a template instructions I found, which were for MinGW).

In any case I built it with the .lib as well, but came across linker issues. Would I need to include the .def as well?

I wasn't sure if the other files / something was missing preventing it from working. The dll was building to a size of 10kb, so obviously wasn't right.

This is all a few days ago now and since moving on, I haven't kept a proper record of what steps I took, so apologies for any confusion.

As I say, I'll probably revisit the .pro file another time. Less of a priority for now as I have something else, but would be good if I can solve it and document it for anybody else looking for a guide to installing a plugin for it.