Whats wrong - tapping and lacking power?

My renault megane 1.6 RT (1996)is making a tapping noise from the top of the engine. The idle is rough, and it's seriously lacking power. It does start up ok, and idles. There is not much oil splashing around inside the tappet cover (from what I can see) but the oil light is not on! Apparently the noise started suddenly, whilst pulling off. Perhaps the timing belt has slipped? Any information would be appreciated. I am about to start stripping the motor!