You could always PM Fletch as he is the guy that will know that engine inside out (probably literally). he may be able to advise if the belt is likely.
I have some cracks in my Aux belt so once Fletch is up and running in his new pad, I'll be on his doorstep with the work to replace mine. But back on topic, get someone to rev the car whilst you listen under the bonnet. You shoudl get an idea of where the problem lies.
Oh, and find a better mechanic. You are paying them, so you don't need to take a crap attitude from them.