there is a hole in the centre of the layshaft put a 1.5mm allen key through it and then use it as a lever to keep the shaft still and try to spin the spur gear, it should be very hard to spin. intact if you tighten the slipper nut fully it should be dam near impossible to spin.
however you are right in that final setting of slipper should be done in the car.
if u are still having issues feel free to pm me!!
