i dont know all the differences but the main one is that the mark ones dont have the holes for a rear fender. One the mark1s i think there was a piece of twisted metal just behind the seat pole but on the mk2s there was a straight piece of metal with 2 holes
Mk1 Contract number: 294/23/S789, WD numbers: C4658474 to C4659686 , Frame numbers: 14 to 1213
Mk2 Series 1 and 2 Contract numbers 294/23/S1649 and 294/23S1946, WD Numbers C5152014 to C5154654, Frame Numbers: 1214 to 3954.
These are the main differences i know of that are obvious