Kevjoyce,
On my 1981 spider the engine numbers are stamped in small letters & numbers on the engine block. Look for a small flat area on the lower left side of block just above and slightly in front of engine mount bracket (between and below the general area of the starter motor and alternator). Mine is a USA car; may be different for British versions (?). I had to clean off dirt and gunk to read it.
Steve in Alabama