I know that I am getting a bit old and forgetful but I am 99% convinced that on a Scammell the clutch brake is on the the input shaft of the gearbox after the clutch, at the back of the bell housing. Remember that in a Scammell (whatever model) the gearbox is remotely mounted from the bell housing.
The clutch stop is there to slow the flywheel and drop the engine revs quickly to enable faster up changes so that you do not lose road speed whilst changing gear.
I know that if you look through any instruction book for a vehicle fitted with a stop the gear changing instruction will be when changing up depress the clutch pedal half way down, engage neutral,clutch up, clutch to the floor (to engage the stop) into the next higheer gear and foot of clutch. There will also be a warning to only depress the pedal half way down when changing down. The reason is that when changing down you need the engine revs to rise to match the road speed in the lower gear, engaging the stop will slow the engine down and defeat the change.