You might find that the ignition barrel has collapsed internally, jamming the whole thing up and refusing to start the car. But before you call in the locksmith, try this: The ignition barrel on this car also operates the steering lock mechanism. If the key has been removed and the steering locked at a particular angle, you can have these very symptoms next time you try to start the car.

What’s happening is that the steering lock is resting against the little mechanism that frees it when you turn the key, making the key feel like it’s jammed. The solution is to pull the steering wheel one way or the other until you feel it move a millimetre or two. At that point, you might find the key will now turn freely.

I'm not sure of the date, but I understand the 7.7mm pitch chain was fitted to engines after number HO53340001. That should tell you if your engine was affected.

It's hard to suggest what might be wrong without seeing the car, so I suggest the best thing is to have an LPG specialist look at it. Give the people at Impco a call on 03 8788 1000 and ask them to check the car for you.