its gonna be expensive for you to follow this course of action, why change a bit that youve been told to by somebody you dont trust?
you will be far better off financially calling in an engineer to fault find on site.
however,
if you changed the "mechanical valve" was that the brass body, or the valve head?
if it was just the head (and im assuming now. op is vague and cloudy !!) are you sure its connected correctly to body ?
no matter which part of the valve you have changed, if you remove the "actuator" the plastic bit, you will see a metal pin in a brass body, if you grab the pin with a pair of pliers and pull it out, or push it in gently ( move it to opposite position it is set at )
you may find a difference in the working of the boiler. try that see what happens, post result. good luck.