Does anyone know where the breathing cap is on a peugeot 206?

My cars not fully firing. I've done all I think it could be, cleaned it out, changed the spark plugs and so on but still no luck. But last time I had an MOT I remember they said it was a heavy breather. So I was thinking maybe the breathing cap needs taking off and the thing needs cleaning out. So how do I go about this? Any ideas?