Ok ive got this as a condition to be entered into a macro that checks the field when the user exits it.
=(MOD(CODE(LEFT( [Stock_TBL]![Stock_ID] ,1))+CODE(MID( [Stock_TBL]![Stock_ID] ,2,1)),10)=VALUE(MID( [Stock_TBL]![Stock_ID] ,8,1)))
This shoud check the two letters against the second last...