Hi
Well, I've finally solved my rattling problem but the cause of it contradicts everything I posted earlier!
I finally traced the noise from the glovebox - around to the surround for the passenger airbag On/Off - then under the glovebox - to the top of the passenger footwell!
Just where the footwell carpet finishes right under the glovebox, there are two plastic lugs each of which have a plastic, foam-backed 'tongue' inserted into them. These are where two separate pieces of the dashboard/bulkhead plastics are joined. I noticed that with the innermost (nearest to gearshift) tongue, its foam backing wasn't poking right through allowing the lug to rattle. The edge of the carpet was just laid over the lug but, was just the right length to hook behind it and then maintain a tension on the lug and stop it rattling. This is what I did and after 100 miles, some on rough roads, the rattle appears to be completely gone.
A proper fix would be to also manipulate the foam through the lug but it's quite awkward to get at so I'm hoping the carpet fix is enough.
I would have bet a fortune that the rattle was from the glovebox itself and it shows how sound resonates from the unlikeliest of sources. It's definitely worth investigation for those with a 'glovebox' rattle.
Mick