I am sure the hard-stamped number under the seat is the best one to use, as the last 3 digits of the VIN number seem spurious at best and we know the story about the number in the wood under the boot being higher than the chassis' due to 'wastage'. I consider mine No.729 (under seat) and this has the inset mounts for the boot liner screws.
For what it's worth, by Ash Frame no. is 795 and VIN Digits 763....
