Depends on the year of your Bessacarr, original Bessacarr's up to and including 96 have a plate by the door, this shows serial number and weights.
The location of the serial number on the chassis depends on year, very early ones on near side 'A' frame leg at the wheel end.
Later ALKO chassis are either offside 'A' frame leg just under the body and forward of the first floor mounting, or further forward on the top face of the same leg so underneath the 'A' frame cover
What year do you think it is? after 91 all are marked on the windows in the bottom right corner of every window