1997 Opel Astra 1.6 16valve

After engine managment light came on I took it for a cam position sensor replacing however on collection it seemed to cure the engine managment light but suffered a big loss of power and poor fuel economy, anyway several days later when I finally managed to hit 60 m.P.H the light came on again, but would go out again, I was told that the timing may be out so I had a new timing belt put on but still no cure! So I decided to plug into diagnostic again, which showed cam pos. Sensor (again) is it possible that the timing being fiddled with, as it was a friend who did this, could have triggered a fault with the cam position sensor, which the garage did, as I am about to take back to the garage, but will be billed if found to be no fault of there replacing of cam pos. Sensor, please help if possible, I cannot afford to keep making mistakes, greatly appreciated.
nick