Many thanks for your replies. Observed a sim session where the instructor disabled the automatic speed brake function prior to departure. In other words the speed brake lever would not actuate automatically only manually. When the crew was given a rejected take off around 110 knots, the captain hauled in reverse thrust and both pilots failed to twig the speed brake had not actuated. The "aircraft" over ran the runway because of lack of max braking until too late.
On the other hand the FCTM requires manual selection of the speed brakes on commencement of an abort and if the pilot forgets to pull the speed brake, then when he applies reverse this will automatically actuate the speed brake. In other words the auto speed brake via reverse levers is the vital back-up to the manual actuation. Makes one wonder why some operators choose to disregard the manufacturers recommendations or do not adequately research why the manufacturer recommends certain actions.