[Richard G]

Is your handbrake leaver fully down to the bottom. It may be that the bump is just causing the lever to come patially into contact with the warning light switch. I'm not sure about the tone though

 

[Richard G]

As a temporary measure
If the remaining part of the button is solid enough you may be able to put a thread onto the inside of it so that you could put a bolt with a lock nut into so that you could push the "button"
I very much doubt either alternative would pass the MOT though