Last time I was there I parked on the apron beside Manx Flyers and made my own may out of the gate. Was picked up by a friend who took me round to the terminal where I paid the landing fee. About £20 from memory.
Getting back in required a phone call to security to open the gate. Only took about 5 mins for them to show up.
Only issue is without transport, its a very long walk around the airfield perimeter to the main terminal. However I believe the car hire companies will deliver your car there - or possibly pick you up.