If it is a 1992 model you should have a log book as the CRIS system came in for that year and it is etched on all of the windows and stamped on the chassis. If it has been removed from the windows and chassis and no log book then you may have a stolen van. When you find the vin for a CRIS reg. van the first 2 letters will be SG as it is UK built,then a number for the manufacturer then a S for single axle and the tenth character will be N for 1992, P for 1993 etc. Pre 1992 it will have a smaller number stamped on the chassis maybe partly concealed by the A frame cover and on a plate by the door. I suspect yours will be a 1991 model year which is why you haven't seen the number everywhere.