The ignition switch contacts can be problematic when they decide to act up. I had a 135 which I bought new in 2004 and had to have the ignition switch replaced - hence a new lockset all round by a local MG dealer about 24 months after I bought the car, sadly no warranty claim due to the demise of MG-Rover. The symptoms were exactly as you describe. Fortunately there is a good source of second hand lock kits about if yours does decide to pack in!