Dave123 explained it correctly.
I'm with Abbey and they instruct to write your account number on the back for the reasons that Dave gave.
Abbey is free, but you have to use a machine, and for this reason you can't pay cash in.(you can but I wouldn't advise it)
Most days I get cheques and i have to put them in an envelope with a slip and write on the back of them I just thought someone else might have a few tips about the stamp.