This has got me stumped. To me it does not sound like a valve problem, I agree with you on that. I have sat and pondered this and thought of something else. From what you have told me about maintaining it, I can only guess that my suggestion has already been thought of. Different fuels burn at different temperatures, thus a spark plug that may be fine for gas, may not be fine for petrol. Have you read your spark plugs as of late? Do they have a light brown coloration or are they black or possibly white?
The other possibility may be the air/fuel ratio while running petrol. This would show up on your spark plugs as running lean or rich, or it could be so far off that they have fouled. This just makes me think also that the valve theory could be right. If you have oil leaking past your valves, it could be worse while running petrol.
Keep me informed. I don't know that I can do you much good, but it is quite interesting to me.
Though gasoline/LP gas hybrids are available in the US, the cost is a bit prohibitive and thus the concept has not taken off. I know GM puts this setup in some of its pickups, as other manufactures probably do to. I am just waiting for a electric car that performs, has a reasonable cost, and can be owned or leased, as opposed to lease only.