Thanks for checking that Frag, i did wonder about flatspotting, but its not a speed related vibration I feel, its road surface variations and imperfections at all speeds.
so had the van at the Tyre shop.
Tyre balance was OK, they inspected it for any freeplay in the suspension, bearings and steering - all OK
tracking checked and a little bit of toe out on drivers wheel that they corrected AND a little bit of camber out (1.9 on the rig, not sure if 1 degs and 9 mins, or 1.9 deg) on the passenger side which has no adjustment, so not corrected.
they couldnt see anything obvious that would have resulted in a degradation of the transmission of road vibration, but my drive home I am still sure it is not as it should be
so next step is to try and get a drive in another transit custom to recalibrate my hands and head. ... might see if ford dealer has a demo vehicle available.
OR if there are any forum members with a newish custom live somewhere between Worcester & Stratford, i would be interested to do a quick swap drive to get a 2nd opinion and a comparison to your van.