Weird Electrical Fault Peugeot 206

I have a 2005 1.4d Peugeot 206. Nobody has been able to tell me what's wrong with my car as this is an intermittent fault and unless it's happening while at the garage, they can't help!

I turn off the car, sometimes it turns off fine, sometimes I turn the key and take it out but the engine stays running and lights stay on, sometimes I turn the key and the engine goes off but all dashboard lights stay on and sometimes the lights go out but then come back on again. Sometimes the lights come back on straight away, sometimes a long while after, (which I've only discover the next day when battery is flat!) When the key is taken out and engine is still running (which is really weird!!) you can only stop the engine by pressing the brake pedal but when you do, you hear a load fan noise. This always cuts the engine but doesn't always turn out lights, they will sometimes go out but come back on and you have to do this a few times before they stay out. Sometimes you have to turn key once to get them to come fully on then turn it to get them off again. Sorry if this is all sounding a bit complicated but it's such a difficult and extensive problem to explain, I hope I've made myself clear!

Thank you in anticipation of any ideas at all!!