Sadly I've been too busy to have a crack at it yet. And/or when I do have spare time it's way too bloody hot to be mucking about with the car. I have a few weeks off work now though so I should be able to get in there in a few days or so.

So I didn't find any obvious noise entry points. Droning is also audible out the left hand passenger side (as reported by aggravated girlfriend) so I suspect my exhaust is just a noisy sod. Might get the mechanic to check for exhaust leaks/mounting issues. The exhaust does also have an annoying rattle at just the right speed/revs.

Check the exhaust over for any leaks.  Second option - electric exhaust valve motor.  They are throttle body like valves, that can be installed towards the end of your exhaust system and hooked into the vehicle wiring (depending on ecu or aftermarket switch maybe required).  At low throttle the valve sits partially open, reducing airflow and sound, at open throttle they open for full flow.  ATS Racing facebook page had a post a while back.

I think you're right Salem, despite my efforts the droning continues. Certainlky road noise is down but it pales in significance. I suspect even the poly rear window may transmit some of the droning.
Just as woofer noise goes out the body shell, I suspect low freq exhaust noise to bounce off the road into the cabin.
  

More on the engine drone. I was sitting at the lights the other day and a big thumper pulled up next to me.
Either a Harley or a Trumpy or something. With my windows closed I could hear and feel the bike idling and it
was all coming through the floor. So although I have put some vinyl sheets on the floor, it is not enough. Thinking about pouring fiberglass resin on the floors to increase the weight....might try one side first. Bonus would be that my subw would cause less doof, doof for my fellow roadusers. There will undoubtedly still some engine drone in the cockpit but any reduction makes the music nicer.

Have you checked out 'sound deadener showdown'? Some great stuff on there.

Thanks 5sgte, I have been there and yes it has good stuff.