!!SOLVED!! - Well identified anyway.
In the end, wrapping foam around the roll-over valve made no difference to the annoying rattle. What it has turned out to be is poor location of the n/s roll hoop within the body cutouts. This means that the large rubber trim grommet has been pinched and worn away, leaving a metal to metal contact between the steel roll hoop and the alloy body. Thus when the body moves w.r.t. the chassis there is a loud rattle/resonating type sound.

Identifying the problem is one thing but I'm now not sure how to fix it.
My rather crude measuring suggests that the hole on that side is slightly closer to the centre line than the o/s one which would account for the problem. (This is a 2014 car by the way. ) If the roll hoop can’t be adjusted, then I guess the hole could be elongated a couple of mm?
Does anyone have any experience of this?