It couldnt be the starter motor because you would be able to hear that? there is no noise.. just 1 quiet tick when i turn the key and then nothing.. mechanic reckons its the brake light bulb.. i mean alternator 
That's why I say starter motor, becuase you can't hear it, only the solenoid click and the engine doesn't start as it's not turning over which is the job of the starter motor.
Your mechanic is no mechanic, the altenator is for charging the battery - you've already eliminated battery/enough starting charge.