Hmmm... MissNancy has kindly checked and pointed out in another thread that the HDRs I've managed to output so far are not encompassing the full dynamic range, for some reason...

So I will need to investigate this... either I've done something silly when I've reviewed the HDRs in Photoshop (at which point I made the lower res jpeg copies) or else there is a further option in the Vue render settings that I've overlooked perhaps...