Hi mate you can just replace the spring in the knob, gently ease out the top of the button with a small screw driver, take out the broken one and replace. £6.50 for spring. the theory with the gearbox is it will fail to engage a built in safety device. Like you wouldn't do it to my motor although did it at low speed by accident and survived.
I also accidentally hit into reverse at low speed today, made a horrible crunching sound but all OK 😅. Needless to say I've ordered a spring from ebay.
I've just replaced the spring. €5,31 incl transport. Slide the ends of the spring in the backside of the knob. The U side to the front. Replace the button first on the bottomside, behind the leaver. Then click the topside in the 2 holes. Easy job.