[Flip]

Out of interest- have they changed the solenoid (starter relay)?

A faulty one would give the symptoms of a click when the button's pressed and often starts failing intermittently as you describe.

I recently changed my solenoid after testing the starter circuit when my bike developed exactly those symptoms and for £30 it's certainly less hassle to change than a starter motor. But to be honest if they know what they're doing they could run 12v direct to the starter motor to see if it spins as it should to eliminate or prove it at fault along with a few other tests to find the exact cause of your problem.